Prothane motor mounts are a good idea when you've modded the engine to the point where the power output is substantially higher. The stock liquid filled mounts have their limitations, but I haven't heard of any breaking. Some longtube header manufacturers recommend them so the headers won't move and hit the floor under acceleration. Mine didn't, but some have. Also, they can add a minor improvement in the transmission's ability to make clean shifts because the drivetrain has less twist. Our shifters are mounted to the body in addition to the transmission which causes shifting problems especially with higher HP output.
The bottom line is if you plan on upping the power and modding out the car you might as well plan on a set, but they aren't something that needs to be put on the front burner.
I got a set of prothane billet aluminum motormounts off ebay for 99$ might as well if u plan on building up the motor, i have seen no drawbacks to them so far.