Sox have only had one off day since the return of Jake Peavy. Combining that with the fact that they did not know what they would get from Peavy was the reason they stuck with the 6 man. In June they have 4 off days and don't play more than 9 games in a row without one. If they stuck to the 6 man, they would each have 6 or 7 days off between starts which is a pretty long lay-off.
I don't believe that the reason for the starters recent success is because of the 6 man. Each one of them are a proven quality starter (other then Humber til this year) and long layoffs between starts could do as much harm as it could do good, especially when your pitching well. I hate to see Humber go to the pen, but I'm sure he will be called on a couple more times this year to start (double headers, long runs without off days, etc..) I would not be the least bit surprised to see them let Jackson walk after the season and plant Humber in the 5th starter spot in 2012.
Humber in the pen also gives a quality arm to eat up innings when a starter struggles. No longer would we have to stick Pena out there to serve up BP. Out of the pen, I could see Humber being this years D.J. Carrasco. Eat up a ton of innings with a mid to upper 3's E.R.A.