A 6-man rotation would just actually replicate like the team had a day off in the middle of a set of games which is a good thing. Also, this would give B-Mac some extra starts, would allow the rest of the starters to stay fresh, but also stay loose. Then in the playoffs, you can use any extra starters for long relief.
If they went to a 4 man rotation in the playoffs, it is hard to say who #4 would be. Jose has came on strong, El Duque is awesome in the playoffs, and B-Mac is a fresh lively arm. They also have their weaknesses, Jose is known for his inconsistency, El Duque is only having an average season, and B-Mac is a rookie.
I guess in the long run, it is better to have 6 quality starters than 1-2 and trying to patch a starting rotation together (see Yankees).