I don't know how story branding makes the game take a back seat. Having these connections and ecology that links these monsters can only serve to make the game richer. DMs who are new to the game can draw upon those connections for inspiration. DMs who are experienced can ignore them or use them at their discretion. Having things linked together doesn't make the game any weaker...
A lot of folks look at the MM, and really the entirety of the core rules as a toolkit. We love to have some suggestions on monster groupings, but would prefer things a little more generic perhaps. As I wrote above regarding the environment/ecology of the Ettercap. Was the 2nd ed, 1 page description a little long winded, perhaps, but there is an enormous amount of information therein that people can use, and hooks to tie monsters together. I would love to see the same info presented on the 2nd ed page of the Ettercap recreated in a modern (Monster Vault?) format and compare it to the original.
Good point. I admit, I don't know very much about Araneas. I really want to keep the corrupted ettercap, I think a constant diet of pixies turning you into some even-more-horrible monster is fantastic. If they go with a non-aranea version and cook up something new, then I'm still on board.
I had heard of the creatures, and probably read them over in previous monster books, but I think there is enough history for currently created monsters to say, well, we have an Ettercap, if they are exposed for a prolonged time to pixie dust, either in general or from eating, they change into a different/evolved form. I think people are upset about the suggestion of using an established creature instead of creating a new one.
An example is that in 4th ed. they have the basic Rust Monster, but if it eats enough Magical items, it evolves into a deadlier variant.
The fandom has had similar issues regarding the re-purposing of the name Archon for 4th ed Angels, and a few others.
I think this is more targeted at convention play and adventure writing. If the writers crafting an adventure for use in Forgotten Realms are working from the same notes, more world consistency is achieved. Orcs behave like orcs, regardless of who's writing the adventure. If the DM running said adventure wants to change it up, they are welcome to. But how often have you flipped through an adventure, tilted your head, and said, "What the hell? Why is a beholder doing that? I thought they were supposed to hate all other creatures."
Again, this goes back to some people, like myself, who would prefer basic suggestions, general trends, perhaps some hints at greater links, in the form a more generic toolkit. But for specific settings, FR, Eberron, GreyHawk, and many more, each have their own ecology in which the monsters will interact. Orcs don't behave exactly the same between FR and Eberron, or Dark Sun (if they even exist there, can't remember) I love to see some general behaviours that the monsters follow so I have baselines when creating my own campaign setting or plunking them down into an established setting.
The 2nd ed monster write up for example, has a section on the Ettercap's general description (even with a picture) a bit of a long winded, but informative combat section, and the two sections, one on Habitat/Society and the other the Ecology which outlines general tendencies, and related monsters
( "Often (40%), 2d4 spiders of some monsterous type" ) and looking over to the spiders page you have Huge, Giant, Phase, Sword and Gargantuan spiders, with several variants of a few of those listed in the two page spread on spiders.
Again, I have no problem with a listing under Hags perhaps to say they like dark, web infuse/infested forests, and can often be found nearby to Ettercap lairs and/or vice versa. Or that Pixie dust is a valuable commodity for illusion spells, that Ettercaps find them a delicacy, and that Hags harvest the dust from Ettercap webs or demand tribute from them. Heck, Ettercap poison is valuable at 1000gp an ounce on the Black Market. I love these types of details, little bits and pieces of hints and suggestions.
This is where I think an Ecologies of Faerun/Eberron/Whatever setting, even as a PDF, would be a good addition. You can say, in FR, these creatures have ties and in this manner, expanding on the generic, established ecologies of the MM, or altering them to suit the setting.
I also think your example of the Beholder proves the idea of the WTF, if you have such a strict, Point A to Point B to Point C ecology listed in the MM, and have convention designed adventure that skews that established ecology by starting at Point Y and jumps to Point ZA and Point 8B, then I think it's even more confusing.
I don't picture this as a give-and-take barter. No, the ettercap skitters out of the woods near the dark and scary hut, where the Hag lives. The hag emerges, and the ettercap fights down the urge to flee. The hag stretches out a bony, clawed hand. The pixie dust is relinquished, and the Hag tucks it away in her pouches. Then she produces three elf eyeballs from a jar and hands them over. The delighted ettercap skitters away.
Ah, but it was presented as a trade in the article, what you have there is Demanding Tribute. Why? Perhaps so that the Hag doesn't eat the Ettercap, or the Hag provides a service back to the Ettercap, food scraps as you write above. These things are low intelligence, but could be taught, but that would seem to me to be more a of localized event, or perhaps a cultural one from the Hag's perspective. Do all Ettercaps entreat with all Hags in this manner, that's what bugs me.
The 2nd ed habitat of the Hag details general interactions with Giants and Ogres, and they like to live deep in forests, so I can see how in a forest environment the two creatures may interact and come across each other. But due to their general natures and individual ecologies I find it difficult to come to grips with the presented interactions as if they were common amongst all Hags, Ettercaps and Pixies (including variants of Hag and Pixie[technically a form of Sprite in 2nd ed]).
Perhaps the ettercaps don't realize it's happening? They don't have a magical progress bar telling them they're at 14 / 20 pixies eaten to assume their new form. Rather it's a gradual progression to a point where they will change. They don't eat pixies to change form, they change form because of all the pixies they've eaten.
I would easily believe that, since they are solitary creatures unless having a mate. But as mentioned above what/where is the motivation to collect pixie dust and trade/give it to Hags on a wide enough scale that it is something that is commonplace to a species...
Now, with all that said, the general plot hook is a good one. Somewhere in the FR is a forest that is tied closely with the Fey Realm. Close enough that Pixies and other Sprites cross over either unintentionally or perhaps intentionally, but a single or pair of Ettercaps have made this corner of the Forest their home and have found a taste for Pixie flesh. A covey of Hags knowing the value of the pixie dust and able to use it to enhance their illusions and disguise spells demand tribute from the Ettercaps. And so, they gather it up and trade it over for those delicious Elf Eyeballs they get so infrequently.. But it's not something I see happening multiple times across a setting, It's a great adventure seed, but shouldn't directly be part of the monster's writeup. Perhaps a Chapter of Adventure seed ideas could be part of the MM that does this very thing, pulls various tid bits together from the monster write ups and ties them together for newer DM's to help them craft encounters. This is something I'd love to see done.