I think it would be kind of cool if users can buy 1 level at a time at 5 dollars or more for games since it would be cheaper, and the download file size would be smaller.
I think it would be good for people who are not sure if the game is good, so they can just buy the first few level to try out for a few dollars rather then spending 60 dollars at once for all the levels on a game which may or may not be very good. Plus, it might save space on people's hard drive for people who fail at finishing a lot of games they downloaded because of a lack of time or lack of skill finishing off a boss from a level...
It also make the release date sooner since levels can be release on a weekly or monthly basis as new levels are made like TV Sitcoms.