I just noticed that unless the FAQ says otherwise MC's can gtg as long as they are not fearless, so Tau and Daemons are the only MC's that can GTG. If they are in glide mode than yes they can do so. Interesting.
I have to look at swoop mode and the FAQ later, but just going by the BRB they can.
However it hurts most of the MC's to GTG anyway. Heck if you decide to GTG with a Bloodthirster you doing me a favor.
However the only models I see this benefiting is Fateweaver and The Great Unclean One. Although I rather Fateweaver in the air, but needing 2's to hurt Weaver in area terrain is not bad, and you still can cast buffs in this state. However with the Great Unclean on he will have a +2 cover save giving out buffs.
Actually Tzeentch and Nurgle princes will benefit from this as well. Heck all I have to do is fly in area terrain and buff all day. With Tzeentch you need a 2 to hurt him and with Nurgle you need a 1. Throw in Fateweaver it than gets rather broken with a +2 re rollable on one roll. This changes some things and gives out options.
The big question is can you gtg while swooping. I would say no since the Faq would put you flying on top the board, while glide doesn't do that. Once grounded I can see you doing so. Going to ground off being grounded seems possible.
To the OP the only army it really aplies to is Daemons. Again Daemons break another rule.
Just saying in general that gtg benefit the Unclean one and that the Riptide can gtg. I know they are not flying MCs that can swoop.