Running an engine rich all the time is not good for it. Fuel can't atomize properly, loading the combustion chamber with unburned fuel. This is hard on the connecting rod (the unburned fuel won't compress). Engine temp also needs to be high enough for efficient combustion. Again, a rich mixture keeps temps low. How an engine is broken in will determine it's longevity, to a degree. Overly rich (read too much oil) won't allow the piston and sleeve to run in properly.