This is one of this random elements that
1)occur once in a blue moon
2)make the game awesome and that much more spectacular
I wouldn't like to see it changed in any way.
And yea, a plane crashing into a tank would do rather good damage to it.
Although I can already guess at your response, in larger team games plane crashes are common and too random. It is not uncommon to have 1-5+ planes crash in a 3v3+ games. I have lost games to single plane crashes, and when multiple hits occur on different critical units. What is most infuriating in these games is that I have built AA vehicles to prevent LoS but they end up costing me the game because of random crashes. Also keep in mind larger maps actually make it more likely planes will crash in player controlled territory or units instead of off map.
Recon run is extremely cost effective because it allows the plane to circle only enemy territory and will not likely get shot down on the first pass. This allows it to give LoS and act as a heavy bombing run.
Planes should crash off map, problem solved, still really cool, but removes random killing of units. If this cannot be, planes should crash at much shallower angles allowing them to fly further and move away from front line units as much as possible.