I personally hate the 'jump up' type of singaround - the excess adrenalin often makes me pitch a song far too high (another bonus for singers with instruments for accompaniment)so I have to stop and re-pitch it.
I think MCs should try to balance giving nervous/words-reading/marathon ballad singers a go, but perhaps encouraging them to change their ways eg: "I expect you'll have that off by heart by next time"
A system that doesnt circle the room allows the MC to avoid more than a single 'go' for a really poor singer, and so keep the session going well.