Well that's inherently what the planes are for and it's not 100% true that you can't get a good amount of regular battles in epic. The problem is variety: There is just not enough variety in epic monsters. I mean if you're like me and you love demons, you're set but if you hate demons or don't want to use them often you rapidly run into issues. Also there aren't a lot of epic creatures worth their name: Poor damage, stats and powers are usually the worst offenders. Epic PCs are pretty powerful and until MM3 and Demonomicon epic monsters were lagging behind by even further than paragon creatures.
It honestly doesn't matter where they pull these things from, but we need more of them and variety for DMs who don't like demons (albeit I cannot fathom not liking demons, but some people hate them!) or undead. But regular every day rank and files are needed. We have tons of (often terrible, but I digress) elites and solos. There's just nothing to go with them.
Constructs, undead and planar creatures are good epic fodder. We could expand things that are already there like Efreets, Djinn, Angels, Devils (there should be a book on devils, but that looks unlikely with the current "direction"), Starspawn and just plain adding new things. MM3 when it added new epic things, like the ancient angry primal spirit race did a damn fine job of it. We just need more things like that and we'll be set.
When you're stuck with pretty much the same options at heroic tier, compared to all the beautiful options available in heroic/paragon (seriously, just think about the number of different humanoid races alone available to DMs in these tiers) you see the problem. As for that last point Stalker, that's why we should have a DMG3 that covers epic tier and expands epic tier adventuring locations (both on the mortal world and amongst the planes). But once again the new thick headed "Direction" of the game probably means we'll never see that and epic tier will continue to be the red-haired step child of 4E support.
Edit: God I sound like such a whinging ninny, but if you're going to make a fully functional and actually fun "final tier" of play for once, I'd like it to actually get some support.