We have: the Monster Manual; Creature Collection; Diablerie; E-Minions; Monsters of Faerun; Legions of Hell; Creature Collection 2; Monsters of Rokugan; Nightmares & Dreams; Jade Dragons & Hungry Ghosts (and those are just the ones off the top of my head).
If you stick to the WotC books, that leaves Monster Manual and Monsters of Faerun. I have content problems (oriental critters, scarred lands style critters and demons and devils are not to everyone's taste) or balance problems (getting CRs "right" for instance) with much of the rest besides the Creature Catalogue because it covers the system for a whole bunch of missing monsters that were staples of past editions of the game and would have liked to see in the core books...and as you mention:
The only direct support for D&D are the Core Rulebooks. Everything else is a niche product and by extension optional.
...and one of those core books, for me, is lacking a lot of the core appeal of D&D - namely cool monsters.
But, to make myself clear, I'm not really saying it should have been replaced by a monster book in particular - more that I think WotC is too concerned with releasing "window dressing" books rather than covering more compelling holes in the product line. For instance, where are the non-megadungeon 3E campaign books from WotC? Nowhere, they don't exist. You can get RttToEE which is a big megadungeon, buy all the Adventure Path series and go for a railroad-a-thon, or wait for the upcoming City of the Spider Queen and go for another crawl.
The DMG devotes chapters to how to create a varied and marvellous campaign, but WotC hasn't bothered to provide an example, and save DMs who aren't interested in yet another megadungeon or railroad-fest a massive amount of work. Put such a thing into a hardback with the D&DG page count, and new players (as well as long-time ones) would have an option of not entering another endless dungeon if they want to play a prepublished campaign.
Hats off to the d20 publishers spending large page counts on stuff which actually saves DMs work, such as the recent city books. They're not to everyone's taste, but far more useful (IMO) than a book that concentrates on something of such questionable utility as god statistics.
Note that this is only my own perspective, tastes and prejudices, and just as you see a world of options opened by D&DG, I look upon it's content focus as a waste of resources (time, talent and money). I can do without D&DG and can see other things I'd preferred to have seen in it's place, but obviously others - such as yourself - would prefer to have the D&DG. The door has opened for entire campaigns based around overthrowing pantheons directly, yet I think that it would have been nice to see WotC get some of the basics in order and fulfil some other promises that D&D implies before branching so far afield of what I consider to be D&D's core focus.
Are you honestly saying you can't imagine any scenarios where you could use material from Deities & Demigods!?
No, I'm not. I am saying that I expect it will have minimal utility to the majority of players, though, and that it will probably sell based on it's novelty rather than it's usefulness. That's just my suspicions though; as I have mentioned, widespread use of the ELH may well prove me wrong.
The past is a guide. Luckily WotC have learned from the myopic sterility of 2nd Ed.
2nd Ed had the Time of Troubles in FR, which featured gods fighting and dying on the surface of Toril. It is widely regarded as an event that many FR gamers wish had never happened.
