It's more fun! Simple as that, better teamplay, better coordination, teammates you can actually trust to help you and to have your back. (Big thing if you're driving an Annihilator in a Siege Defense.That mech makes you slightly paranoid!

Teammates that most of the time actually come and push with you, and not just come up with you until they are first shot at and then retreat. If you commit to a push in an assault you are commited, yeah you take the damage but that is so the team can win.
Nothing is more annoying to see a team that just wastes that sacrifice, by just bugging out as soon as the first shot comes their direction.
Oh yeah and people that are actually smart enough to use comms and coordinate with each other. Ok due to me almost exclusively playing FW, I'm still stuck in Tier 4, but it's annoying to see how little people talk with each other in QP.
Well, until they are dead that is, then they are happily bitching about how their team sucks, and how they knew it was a trap.
Yeah why didn't they open their mouth and talk about their knowledge before? Or type?