Helis are low armor units. Don't expect to make a long siege with them or do a lot of damage. But they are fast so they are perfect :

1- for supporting tanks and give them advantage on the field.

2- for stressing/disturbing/distracting the enemy defenders : attack a building, go backward, repair, attack another building, go backward, repair and so on. Make them mad with all these useless alarms...

3- for attacking the harvy. Oooooooo yes the harvy is so slow, and your heli is so fast... You can do it in multiple attacks to make the necessary repair...

4- defending the base. With your heli the most useful thing u can do i think is to defend your base : damage the rushers (flamers, apc, cars and helis ) and warn your base about any incoming threat. With high seeing and patrolling u can see them coming. Don't forget with your heli to search for beacon too.

5- As your heli is a flying unit, u can repair it on unexpecting place to make it harder to steal.

6- In a team of 10 players, 2 helis are a max. More apache/orcas are useless, unless for the mood (Oooooo yes i love seeing a 5 aicrafts squad, this make me feel in "apocalypse now" ). By the way, don't expect to become MVP if are in a heli during all the game.

7- In a heli, fear the buggy/hummer ! They re fast and can chase you ! With a buggy i ve destroyed a low altitude orca... (not tested with a high altitude). Fear the infantry too, especially snipers.

8- on air maps, with all these ramps and hiding places, beaconing (even as GDI) is really too easy. Go ! You can do it a lot !