A more frustrating "dissident companion" is Ashara. If you're a DS SI she's the equivalent of a LS JK's Scourge- can't change her and probably don't want her on your ship. The fact that a SW can have Jaesa go one way or the other likely means the thought was there, it just wasn't further implemented.
Another reason for Scourge to stick around is the stock line all non-LI companions give in the end (I'll-be-here-for-your-legacy-etc). If you're the least bit DS- and take the DS convo choice- his assertion that he'll train your child(ren) is very compelling. What else does he have to do? As Ben pointed out, he can't exactly go waltzing back to the Empire...and do you really see him becoming another Praven, even if the Jedi would have him (huge
'if')? In the legacy context, the JK may be the only means by which he could
return to the Empire- via the JK's offspring.
(edit: the aforementioned final conversation, if you're LS and choose LS, still has him claiming that he'll hang around to see to your kids but it takes a more menacing tone; he implies that as a Jedi you're a waste of power and that he won't allow the same "mistake" to occur again)
I tried to think it out- hope that made some sense, the caffeine's just not kicking in fast enough today