Importing a roster makes a ton of sense to me. In fact, it would be one way to to allow you to play the 2013 schedule next year instead of having to start in 2012. You could sim through 2012, then overlay the 2013 roster over your franchise at the start of the 2013 season and have correct team rosters, ratings and 2012 historical stats.
If the import simply updated players to be on the correct teams, changed their ratings, and brought in the historical information in the roster file it would be great. As updated rosters came out during the season you could keep you franchise rosters current with reality. As long as the roster file historical data ended in 2012, it wouldn't mess up any of your 2013 (or beyond) data..
I run my franchise in parallel with the real season and only play the current season and then start a new franchise the next year. So of all the mod suggestions I've seen, this would be the most valuable one for me.