Improves solo PC longevity but doesn't increase the class abilities as a side-effect - you still have the fun of leveling up and gaining power but your risk of dying before you ever see any kind of growth is mitigated.
Simple to implement
Allows you to play published adventures with a single PC with little or no modification of encounters
Works with your existing character generation rules
Works with existing game mechanics
If you are wed to the idea that the game is one of nobodies struggling and dying on their way to fame, fortune and hero status, this changes the very nature of the game: they are a hero of sorts at the outset, in the sense that they are able to withstand blows that would kill them in party-of-PCs game quite easily.
More thoughts on Solo Heroes here: http://tabletopdiversions.blogspot.com/2014/02/black-streams-solo-heroes-review.html