They are good enough to compete for the division:
If Buch stays healthy
If Masterson stays healthy and gets back 4 mph on his fastball.
If Miley doesn't give up a couple of dozen HRs and can get his ERA below 4, and
If Kelly can be more than a 5 inning pitcher.
When you have this many "ifs," a few of them will not turn out well. When 2 or more don't happen, things tend to go south. So, I disagree that this rotation is good enough on paper to compete. They can compete "if" 4 of them can turn around last year's performances.
When I start hearing, if, if, if, if, I think that means trouble.