Imoen speaks literally a grand total of twice in the original game; The conversation where she introduces herself, and another when she joins up with you after leaving Candlekeep. Beyond that she says nothing save the default command stuff, so if you found her annoying... well, that's mostly just your personal preference. Didn't like the voice actress I take it?
The reason she has so little dialogue in BG1 was because during testing the developers realized that not only were there few thief characters in the game but almost none available early and that the early game on leaving Candlekeep was being touted as too difficult to travel alone by playtesters. Their solution was to whip up Imoen as a childhood companion for the player character late in development, two birds with one stone and all that. What the developers hadn't anticipated was the characters popularity nor that her stats also happened to have made her an idea thief/mage multiclass, one of the most useful combination in Baldur's Gate. One might say the reason for her popularity was that she was so damned useful nearly everyone had her in their group and, having so little dialogue, projected their own stories onto the character.
Thus she returned in BG2 as sort of an idealized fan version of the original character and with newfound importance to the plot. And yes, I do happen to quite like the character. Since she says almost nothing in the original game its difficult to find her annoying, while in the second I rather like the personality they gave her. She's also got some great quips with various group members; I especially love how she taunts Edwin endlessly if he's in your group. It's genuinely hilarious. Her dialogue with Sarevok in Throne of Bhall can be fantastic as well should you make the right choices.
So useful character in BG1 and entertaining one in BG2. Not bad at all!
I know it was a rhetorical question in that you weren't really expecting an answer, but there you go, that's why Imoen is there.