Changing motor mounts with engine in car

I doubt that rubber mounts are cheaper at the factory.

Don't forget about frame flex. If both solid mounts are on the K-member and a rubber is on the trans, then the K and engine make a solid unit, and the trans can flex. So that probably is quite survivable on the street. The trans mount must be seeing some good movement if there is some high torque and good traction at the strip.

If you are on rough roads a lot, then I'd surely use rubber in at least 1 point. But in rallying or rough roads or trucks, you had better use rubber all around with straps if things are really rough I've bent a K-member-like frame member just hitting a huge buried rock hard in a rally in GA years ago. If there had been solid motor mounts in there, it very well might've cracked one side of the block.