I think Ronon would have been great as a recurring character, but he had no place on the team for all of season two and three. By season four he is no longer pulling a gun on people in an instant, but that still doesn't explain why this guy was allowed on the team all those missions previous.
Better to leave him as a runner. A character who wants revenge against the Wraith. Sometimes this puts his interests in line with our heroes, and sometimes it pits him against us. We never know what the case may be.