Teamwork is always important in airsoft, and a large factor in having a good team is communication- I'm not saying there won't be the usual game where no one works together and everything goes pear shaped, but even in that sort of scenario you can still direct people to useful positions, co-ordinate plans, call out where enemy positions are etc. Now, when you're wearing a gum shield, it would be kind of hard even to do that- and over a radio you'd be misheard and misunderstood by the guy on the other end.
As well as that, you wouldn't have the lower half of your face protected except your teeth. Idk about you guys but I'd rather not turn up to college/work after a skirmish looking like I'd been attacked by a swarm of angry hornets. Some people may like scars and not care about how they look after.
IMO mesh masks > gum shields, mainly because they're more comfortable, easier to communicate (and thus improve your team's chance of winning) and they protect your entire lower face- not just your teeth.