[TaSSaDaR]

Purely from the RP point of view, you ought to have Imoen in your party when you.. hm.. encounter her. spoilers ahead:
.
.
.
.
.
.
.
.
.
.
.
.

She's your sister, do not forget this. And also, I hit BG I with her, she proved herself to render decent help, and she's so cheerful! By the way, Baldurdash fixpack fixes her thieving abilities, so they become sufficient. And, finally, personally I would prefer a party of no ugly races (such as half-ogres, gnomes, dwarves, halflings etc), and this is the most important thing about the decisions Imoen vs. Jan.