The engine was tuned to that specific muffler.
Running without baffles will change the back pressure which may cause the engine to run a little lean and thus you run the risk of overheating & seizing .
All depend upon how fine the engine was tuned.
Price of a new muffler is less than the price of a piston so it is a bit of a no brainer.
A few short runs probably wont to too much damage just be very weary of the power output till you get the new muffler.
If you feel the engine labouring or loosing power for no apparnt reason, shut it down pronto.