I don't know anything about Jennifer's alleged drug abuse. And until someone shows me a picture of her with a needle in her arm, I'm not buying.
Having said that, IMO Kes and Harry were the least interesting regular characters, with Chakotay nipping at their heels for third place. I figure the producers weren't about to decimate the cast so they tossed three names into a hat, and Jennifer lost.
Voyager was in Borg space. So having Seven of Nine join the crew worked so much better. Her history as a Borg and her journey to recovery of her humanity was far more interesting than watching Kes handing hypos to the The Doctor, and setting Tuvok on fire (tho' I have to admit, that was pretty cool).