this is a little bit of a theroy, but i think that it is pretty well founded on experience and observation.
there was a discussion in the thread that high pitch counts injure pitchers not high innings pitched totals. i don't neccesarily agreed with this. although there has been plenty of work on pitcher abuse from high pitch counts, there has been less on innings pitched totals. but there has been work to determine the year to year percentage of work increase that is acceptable for a pitcher--current theory has it at 30% inning increase year to year.
i don't agree with either idea really. my thought is that the number of maximum effort pitches is what is important for arm health. lets take livan hernadez for example, high pitch counts but very durable. he is not throwing every pitch at maximum effort. he is saving some in his tank for high leverage situations and innings. to go to a four man rotation and increase the number of innings a pitcher throw will increase the number of high leverage innings he pitches and inturn increase the number of max effort pitches thrown during a year. i think this increases the chance of injury more than a pure pitch count vs innings pitched argument.