I think, depending on the system in question (real life), some of the larger LR SAMs do not or will not lock on to something that small. Either by design or happenstance system design flaw (SAM-2).
OTOH: I know the Russians made a point out of using AAM's to shoot down at least one Georgian UAV prior to their invasion.
http://www.youtube.com/watch?v=dNpABtIKERg
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Game-wise I'd much rather burn up as many as a dozen UAV's exposing the AI's positions before I move a single ground unit. Ground units take objectives, UAV's cannot. UAV's are much cheaper then MBT's purchase points wise.
MASSIVE
OVERKILL,
R