So your argument really boils down to: If a game store owner tells people that they can't use something, they'll probably be less likely to buy it?
I'm pretty sure the argument was that the
back of the book states that it's for use with HotFL, HotFK, and RC. I'm with Dice4Hire on that one -- if you aren't the sort of D&D player who follows what WotC says online, how would you know that HoS is compatible with PHB1-3?
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I think the big problem in this thread is that people are using "compatible" to mean two different things. There's compatibility from the DM's perspective and compatibility from the player's perspective. The two are
not the same, and conflating them confuses the issue quite a lot.
Essentials and PHB 4E are very compatible from the DM's perspective. It doesn't really matter if some players in your group are playing Essentials classes and others are playing PHB classes.
Essentials and PHB 4E are somewhat compatible from the player's perspective, but not fully. Even the latest multiclass and hybrid rules update doesn't make them fully compatible. It's certainly a big step, but it's clearly factually inaccurate to say that they're "totally compatible" in this context. It doesn't matter if supplements in past editions have broken compatibility with mainline classes, it still isn't accurate to say that Essentials is fully compatible with the PHBs.
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Part of the reason some people are upset with the direction Essentials took goes back to the statement by WotC in the run-up to the Essentials releases that this would "be the baseline experience for the game going forward." That the Essentials format would be the new format for everything, not just the main Essentials books. So far this has been the case -- the HoS classes are are in the Essentials format, and are mostly either Essentials-style classes or options for Essentials classes.
This holds true across the game's presentation.
Your options at the online CB main screen are New, Custom, and Load. The explanatory text next to the New button informs you that this will only allow you to build Essentials characters. If you want to build a character using anything other than the Essentials line, you're into "Custom" territory. Why not have one New button and have Essentials be an option in that popup alongside Encounters, FR, DS, Eberron, and D&D Home Campaign? ...because Essentials is the "baseline experience for the game going forward."
I've been playing a battlerager Fighter since about the time of PHB2. Now my character sheet says Weaponmaster, because WotC decided that the entire group of pre-Essentials Fighters could reasonably be lumped into a single subclass. Meanwhile, each Essentials Fighter is a subclass unto itself. How does someone building a Custom character in the CB for the first time know that Weaponmaster has an order of magnitude more options than the Slayer or Knight? They don't, because all of that content is stuffed into a single subclass, implying that the Slayer and Knight are each individually on par with the entire set of Fighter content that came before.
Individually, these are annoyances, not huge problems. Taken together, though, they suggest an underlying objective to obscure pre-Essentials content from players. It seems like they would be happy if everyone would just play Essentials. Fortunately, there was enough of an outcry from people who want more support for pre-Essentials classes that Mike Mearls finally told us explicitly in Rule of Three that there would be further support for them... in
Dragon. Not on store shelves, not in the same way that PHB2 and 3 gave us a lot of new and interesting options. Support for the AEDU classes is being relegated largely online, with the occasional snippet in a post-Essentials book like the new Cleric powers in HoS. Because Essentials is "the baseline experience for the game going forward."