Makes it look old
Makes it harder to notice future oil leaks.
May result in a potentially-harmful low oil level.
May give a nice burning smell from the exhaust manifold.
Common place for leaks seems to be distributor O-ring seal on outside of distributor body, which sometimes makes it look as though the end of the cam cover gasket leaks. Also take off distributor cap and check for oil inside - my friend James discovered that it can actually stop the car if there's too much oil in the distributor (I'm not sure how, but it did). The shaft seal is the culprit if there's oil in there. Easy enough to change, but can be difficult to get the new seal.
Meanwhile you may like to try removing the rocker cover (not that hard) and cleaning the surfaces and the gasket with meths or engine degreaser, then use a smear of Loctite Master Gasket (non-silicone translucent red goo, sets anaerobically). But for all this trouble (hour or so), you might as well fit a new gasket.
-Alex
