The reason I thought so, is that they have it in another BF2 mod, AIX.
Therefore I know that it at least is possible.
In addition to that, I think it would be realistic, as it is possible to turn them of whenever you want to in RL.
It is not always desirable to be seen, or in this case heard, by the enemy.
Doing a 360 scan in the AA, it can easily lock on to something.
Something that you can let get close to you, slow down, prepare to land, and then send a few bullets flying its way in stead of using a rockets.
Some may argue that you can always just look to either side, to make it break the lock, but that is not the best way, as there might be another chopper there, that has seen you, resulting in your death.
In addition, when then returning to your target to take it out, you will once again have to aim, giving it a head start.
For the enemy, that would mean that even if they don't hear a lock on their bird, it may not be safe.
Imagine that it's at the start of a round, transport is flying back and fourth all the time. Then your AA shows up, gets UNlucky, and locks on to a chopper flying back to base. What do you do?
The pilot will most likely warn his team, resulting in a HAT showing up and taking you out, or the choppers will find a new route. Possibly both.
If it was possible to turn off the radar, the bird would not be alarmed, and it would come back just a bit later, with important supplies, + possibly some troops + it would delay the use of that chopper even further.
This would mean that the importance of a defined front would be extra clear, to keep the enemy from doing this to your supply-lines. (teamwork)
The effect of an ambush could also be increased.
I suggest using the key 'F' to swap between these two modes, as with the tanks and their different ammo.
As said, I know that it at least is possible, since it has already been done in AIX
