Greenhouse gases could also create a ozone layer
http://www.science.nasa.gov/headlines/y2001/ast09feb_1.htm
NASA scientists believe that supergreenhouse gases could warm Mars in just a few hundred years, then create an atmosphere which would have a radiation blocking ozone layer. The solar wind erodes the atmosphere so slowly (doesn't really matter if it's 1 million years or 100 million years) that it could easily be maintained.
I'm not sure about the frequency of the really big asteroid hits, but it stands to reason that an atmosphere on Mars would certainly cushion the blows of the occasional Tungusta sized rock that comes along.