I think these are perfectly valid concerns. (Strangely, a mod I'm working on right now addresses exactly these concerns, including Imoen. I *think* you might enjoy it for the roleplaying reasons when it's ready).
I think I would decline Kimmuriel's offer, too, if I cared about my soul, or Imoen's soul, or anything at that point. My protagonist feels rather empty, though: she vaguely knows that something's wrong with her but doesn't really care.
But what if...
- you've lost your soul but you aren't feeling any different, really, and you really don't think it's that urgent, despite Keldorn's/Viconia's urgings?
- you've stopped caring, basically - you know on some level that you must get your soul back, but such things are no longer relevant to your soulless persona?
- you're intrigued by drow and you believe your time in Ust Natha was the best few days/months of your life, and to hell with everything else?
- you always enjoy the smell of new adventure?
- you're a girl who liked Kimmuriel(THAT voice!)?
I think these are interesting enough reasons to journey to Menzoberranzan. And if not, there's one more: before going to meet Kimmuriel, you DON'T know that you're going to go to the Underdark(roleplaying, yay!) When you meet Kimmuriel, he explains the situation, but he doesn't allow you to say 'no': if you do, he shrugs and teleports you anyway, just like he's teleported Artemis Entreri. Drow will be drow.
P.S. There's no spoiler info in this thread yet, but just to check - you've got access to the beta forum, right? If you do, I would like you to post bug reports and issues there, so don't lose any valuable info. (And, yep, spoilers).