If you have SoA only all NPCs max out at about 2.75mio XP IIRC
Therefore pure (Edwin) and dual mages (Nalia,Imoen) excel over multi-class (Aerie,Jan) because they can learn higher spell lvls. Imoen is a quite useful character as
1.) she has about all the basic thief abilities you crucially need (detecting traps and lockpicking) and you can of course imporve that with potions
2.) she makes an excellent mage but can also use some mage-forbidden equipment (e.g. Bows, Swords).
3.) She excels over Nalia because she has a better thief portion and better looks
The reason Imoen is often considered weak is that once you rescue her your party is often higher in level than she is. Therefore she tends to be "the one with the low experience". But try a game with lots of party reforming you'll find that some other characters suffer the exact same setback at some point.