It would be nice, but USB only does 0.5A, which, as far as I'm aware, isn't enough to run the Pandora, let alone charge at the same time (I recall Squidge saying this somewhere).
Ergo, a DC socket is invaluable, unless somehow the USB port completely breaks standards, but I imagine that would need a custom part. Since a custom part needs to be researched and mass produced before it can be used, that would be expensive (I imagine). Furthermore, the Pandora still wouldn't be able to charge from a normal USB port on a PC - that would only be supplying the standard 0.5A of current (could charge, but not whilst on). So you'd still need a DC lead to charge it anyway.
Add that to the confusion that would result (I plugged it in to charge on my PC and 20 minutes later it died - my Pandora sucks!) and the fact that there's plenty of room for a DC input on the box (so we're told by MWeston), and there really isn't a positive reason to do this. It would be expensive, confusing, and unnecessary.
Only because USB doesn't provide enough current, of course. Otherwise, I'd be with you. But it doesn't, so I'm not.