I'm glad to hear that you'll check the Malcanthet article out anyway, Flexor; I'm sure there'll be something in there to tickle the demon-fan for anyone, as in it DOES introduce some new demon lords and adds a new monster and so on... And at the very least, if Malcanthet is your least favorite demon lord (she happens to be one of my favorite, so I'm pretty tickled that I finally got to write this article), you're set in not having to worry she'll be the focus for the next one!
As for the question DMH posted... first off, don't feel too slighted that no one from Dragon has answered the question yet. They've been in Deadline mode over there the last few weeks, and I suspect haven't had much time to track threads here.
In any event, although I'm not on the Dragon staff, I think I can answer the question for you by listing several reasons why you might see monsters with similar themes presented over and over:
1: Not everyone has every book. If you don't have MM2 or Libris Mortis, the idea of an undead graveyard might be new to you.
2: Sometimes when you see repeated monsters, it's the designers attempting to "fix" or refine the previous monster. A lot of the creatures in MM2 had design flaws; they were built during the earlier days of 3rd Edition when not even us designers knew all the ins and outs of really high-CR monster design. This is the reason you saw the more powerful demons and devils get facelifts between the 3.0 Monster Manual and the 3.5 Monster Manual.
3: There's a LOT of monsters out there, and there's really no way for one person to keep track of them all. Duplications in theme are guarenteed to happen now and then.
4: It's sometimes handy to have multiple monsters with similar themes. By using these various undead graveyards (of which there are indeed a lot of), you can generate an interesting adventure set in an ancient necropolis where it's not the dead bodies buried there that attack the PCs, but the graves themselves. By having several different variants on the same theme, you can set up sevral encounters that all build to support the adventure's conceit but are more interesting than just fighting the same monster over and over.
I can certainly understand the dismay about seeing a monster that reuses another monster's themes or niche in the game. My personal pet peeve in this arena lately is the varag, a monser WotC introduced in Monster Manual IV. This monster is, in almost every way, an exact duplicate of the bugbear (down to being a goblinoid). When you have a bugbear already... why do you need a varag? Of course, in this case, my annoyance is further compounded by the fact that the varag not only does everything a bugbear does, but he does it at a lower CR score than a bugbear (whcih either means the bugbear's under-CR'd, or that the varag is one of the toughest and most unfairly buff CR 1 monsters in the game), but also that we've got four pages of the book wasted on a monster I basically already had.
I won't be using varags in my game, but that doesn't mean that they aren't valuable additions to someone else's game. Same goes for monstrous graveyards.