AbdulAlhazred
Legend
Yes they do, but not as much as multi-classing. A theme brings in a few powers to blend with everything in the game. MCing can blend class features, 100+ powers and a slew of class feats, which is just so much more to balance. I would definitely prefer wizards design a 'theme' for each existing class as a multiclass.
Plus, themes don't detract from another choice - everyone can take them. With multiclassing, you're choosing either to multiclass or single class. If multiclassing becomes a more optimal build, then all single classes consequently suck.
Except that existing 4e MCing is very light, and thus while technically your character is 2 classes you rarely notice that much. Especially if the 2 are similar, which they usually are due to mechanical considerations. So who cares if MCing is 'more optimal'? Besides a blanket statement like that can't be true for all cases by far. There is no danger of a single class becoming some kind of ultimate class to MC into due to the way 4e works.
The problem with using a 'theme' as an MC is a theme is a small thing. You attach a few powers to it, maybe a few feats, etc. There's no way that kind of 'one-size-fits-all' taste of another class is going to meet more than a very small percentage of people's needs for MCing. It may well be that themes as used in DS and whatever may come out later will assuage the need for a lot of players to MC, perhaps. Themes are fundamentally intended for a different purpose though, or at least a specific purpose which isn't always why you MC. You may in fact want to do both.
I think the existing MC system is actually pretty good. It is just too feat intensive and doesn't usually give you enough back in return to make it worth going far with. The fact that it is all based strictly on power swaps actually pretty well keeps things in line and yet makes it interesting. I still think 2 feat MCing is about the optimum.