be it from an offensive or defensive POV, i can't really see an actual difference.
should you stack healing stats, the difference, while it does exist, won't help a bit if you got with a half-assed group who can't manage to dodge properly, not to mention skills which heal others never heal as much as the self-heal. defensively (in a support way) speaking, the impact is at most negligible. when it comes to tankiness, people could make use of wayfarer gear (tough/vit/mf) and be quite bulky you wouln't even come to think he's using mf gear (as you usually don't expect a tanky char to have the highest dps anyways).
if you can manage to keep yourself alive no matter what, stacking more vit or toughness is kinda overkill.
offensively speaking, traveler gear provides pow/pre/mf, which if you don't run a condition build, doesn't make that much difference with berserker or rampager gear either, ie no dps loss. as for condition based builds, how much of a difference does it make? i sure can't tell as we got (hopefully) no dps meter IG.
Also, with no IG fight with a gear check mechanic or enrage timer, the only times it can actually end up gimping a group is when players who use it aren't good enough to keep themselves up to start with, which brings us back to my previous post, ie in 99.9999% of those times, it probably wouldn't have made a difference if they switched to more defensive or support gear, such as knight or valkyrie or whatever.
decent/good players who find themselves in trouble will usually switch their gear if they feel they need more survival/dps.
so where's the bad about being selfish when it makes zero difference?
as last resort, if things are turning wrong and the dude(s) doesn't want to switch gear to fit your needs, just kick him out of the party and well, problem is half solved already. though i've never kicked someone in gw2 for his gear up to now.