Fullshare was once the default and it should be again. Without it, too many games are decided by disconnects.
If you are dumb enough to kill the lowest rated enemies first in a fullshare game... well... you will tend to lose rating xD
If you join a Setons lobby and Full share is not enabled while not as such advertized in title... you can be pretty sure the host has a team full of buddies with some optimized strategy to whack 1-2 of your teammates...