I would like to see this feature implemented as well. The only problem I can see is the spawn point bombing that would occur. Suddenly, it would just require 4 bombs to disable a small field (both ends of the runway, and one each in front of the hangers that spawn)
So if you hardened those points, so they could not be cratered, it would work. I think the trees, and killer sheep would deter non taxiway/runway takeoffs.
And for those of you who think hitting a sheep on takeoff would not do much: While in the Army, our unit was deploying to Japan for a training excerise out of Honolulu. On takeoff, our chartered 747-200 struck a single goose dead on the nose cone. Not only could we clearly hear it throughout the plane, but when we landed in Guam for repairs, the whole tip of the plane was caved in so that a 6 foot man would have no trouble standing upright in the dent.
So while our ground vehicles would have no problem makeing short work of the livestock. They should pose a grave threat to planes.