The reason that 3 RP only rule was implemented was that unscrupulous owners could manipulate rules to get 400+ innings out of relief pitchers.
Basically the Starter/Relievers would be put in the 5 man starter schedule and the rest of the roster filled with starter only's, and one premier relief pitcher. Strat won't use starters as relievers if they are listed on the rotation, and thus abuse of the system was born.