I always find myself using either Yuffie or Vincent in my squad, but I tend to lean towards Yuffie for several reasons:

1) I really don't like the lack of control over Vincent's limit breaks, sure they were powerful and lasted more than one turn, but I'd like some way to control it.

2) I really dislike Vincent's ultimate weapon, Death Penalty. I believe the lower the HP, the more damage it does (the opposite of Ultimate Weapon) and since I always like to have my characters at full or near-enough full at the start of a battle, the two really don't go well together.

They are both useful, because of their long-range, without them you'd only have Barret for long-range. Vincent's story is a lot more compelling than Yuffie's, though not the greatest I've seen. I'll give Yuffie an 8 and Vincent a 6, so I'll vote 7 as an average.