I've asked numerous EMRs ad EMTs both civilian and military and the concensus seems to be that their mouth won't go anywhere near somebody elses unless they have a mask with a one-way valve.

It only takes once to catch something nasty...and not everything can be cured once you get it. Not to mention the fact that I can't imagine what it would be like to get some vomit funneled into your mouth through a mask that doesn't have a valve...that's reason enough for me.