A) Angel has experienced the Savage Lands many times. This isn't some 50x50 foot cell they're brawling in. It's basically a small country. The native inhabitants to the Savage Land would eat Green Arrow alive. I realize Oliver has survived Jungle-situations before, but this is like no jungle he's ever traversed. Full teams of X-men have struggled to make it through unscathed. The ability to fly and avoid 90% of the complications this place may provide would be invaluable. Simply hiding in a tree does little for you here. He could just head for a recognizable safe haven and come back two days later to see if Mr. Queen is arrowless, much less alive.
B) Stan Lee once stated that the point to Angel wasn't just his flight. Characters like Iron Man, Human Torch, and Ms. Marvel can fly much faster than Warren, but his strength comes in aerial combat. Due to the sudden directional changes and 'darting' his wings provide, no Marvel fligher without the power cosmic or the like can out-flank him. Due to his reaction speed and the fact that arrows don't quite move at bullet speed, it would take a point blank shot for Oliver to hit Warren if he's airborne.
C) Among additional underrated powers Warren possesses, he is a powerful hand-to-hand fighter as trained by Wolverine, Black Widow, and Hercules. Also, when speaking of his teenage combat training, Warren has said, "My father spoiled me with more than money."
His wings grant him superhuman strength feats. Though Green Arrow may still be the premier street brawler, Warren's two additional appendages have a reach diameter of 16 feet from tip to tip. The punches delivered from these are capable of shattering human bone.
He has a healing factor that would make arrow stab wounds a mere annoyance, and reduce poison/stun/electrocute effects to a small fraction of the duration that normal humans feel.
Keep in mind I do think Green Arrow is the superior Hero, but this is simply a situation where he has run into a 'hard counter'.