The real draw back is you can not hear if there is something wrong with the engine. Never run a rebuilt engine without a muffler, it could be tearing itself apart and you would never hear it.
For short drives on a worn engine, you shouldn't have any problems. Add a foot of pipe if you really worried about it. Exhaust pulses don't travel to far up the pipe, and even if they get to the valves, the valves are not going to be hot enough to get damaged. Pulling a full load up a hill is a different story.
Joe
