In game modes like trials or skirmish you'd want a high impact sniper because they can 1 shot revives. I've heard that patience and time can do this with aggressive ballistics but I haven't tried this
With a low impact, high RoF sniper rifle, missing the headshot and hitting the body isn't really an issue, because you'll be able to fire that second shot almost immediately without any shot correction or them having a chance to run.
So there's that.