Well mainly you don't want to stand close enough to allies that he'll get a multi kill. I know that if there's only a chance to kill one person it's a waste barring certain circumstances (if you have heavy ammo or are using a super I'm going to stop you). Nothing makes me happier in control than having my super and seeing one of my points going down faster than a drunk sorority girl. It means there's at least 3 people there and a multi kill is incoming.