If someone's carrying a SAAW, I hope they're not walking around with it out. Maybe it's just me, but if I'm packing a SAAW I default to the SMG. First realizing there's an enemy UAV, from the time the enemy commander clicks the UAV button, to the time you realize there's a UAV in the sky takes at best 2-3 seconds. If it's directly over you, it'll take alot longer to notice it's there, gunships approach from an angle, so you wouldn't be looking directly up. If it's further away such that you can notice it easily, then it's a distance where it's pulse, if it's friendly, may not be on your minimap. So you open up your normal map, quick glance, go back, let's give that 1 second.

So you bust out that SAAW, which takes about 1 second. Aim at the UAV that you've determined is enemy, and wait for a lock, which takes about 2 seconds at best. Fire off your two shots and continue holding click for a strong lock, and wait for your shots to get to the UAV and hit, for a total of 5-10 seconds? (we'll assume 5 for best case scenario). If your first volley doesn't destroy it, then you now have to reload which takes 5 seconds, get a lock which takes 2 seconds, fire a volley and hold click til impact for another 5 seconds.

Total all that time up, and it takes about 23 seconds to fire off two volleys at a UAV. Seeing as a UAV lasts for 30 seconds (correct me if I'm wrong here). You can shave off 7 seconds off the end of a UAV. By which point the damage is already done.

Or you can spend those 23 seconds planting Motion Mines or shooting your pellet gun at incoming infantry, or repairing a vehicle.

If you're at some back flag, with no combat going on around you, then by all means, destroy the UAV. But situations like that happen rarely, and usually if you're at a back flag defending, you'll be too far away from the action for the UAV to show up on your screen. Unless it's directly over you, in which case you won't notice it's there at all.