Fair enough. I'd be interested in an explanation of why aberrants seem to love hanging out underground so much. Does it feel like home? Do they not like the sunlight? Or do they just have a macabre sense of feng shui?
I haven't had this question come up in game, but I suspect the answer is two-fold, from both an in-world explanation, and a game-design explanation.
In-World: The underground is much less populated, harder to get too/more defensible, and easier to hide in. Aberrants are supposedly less populous than normal races, and so they would want to remain out of sight as much as possible. There'a semi-evolutionary line here: Aberrants live underground to avoid being slaughtered by adventurers, so they adapt to live underground, flourish, and become a high-level threat while in their natural habitat.
I should also note that, at least in 4e fluff, abberants are entirely limited to caves. Foulspawn could just as easily wander the country-side, aboleth could live in sewers, or a beholder could running a religious group from within a city. That's just not the norm as those abberants I just mention get killed faster, leaving the ones underground alive and still reproducing.
Game-Design: Delving undergound is seen by many as more difficult than adventuring above ground. Its scary and atmospheric, so it makes sense that you'd put some of the weirdest, scariest things down there in the depths. Underground can also be seen as an alien place. Have you ever been cave diving a mile below the crust? It gets freaky, and the creatures down there are fitting, so its natural that you'd place similarly alien creatures in that environment (in the same way deep-sea creatures are equally alien and weird).
Agreed. Especially the power swap feats -- so lame! Luckily, C4 solves these problems.
One of the house-rules I use is that multiclassing automatically allows you to power swap. When you level up, you can pick a power from your MCed class, though you are limited to 1 multi-class power per type (utility/encounter/daily). Its been very well-received and makes multiclassing much more common.