I don’t think that the Rule 5 draft is very good for players. Take Jeff Bennett. Bennett was a guy with a live arm, drafted out of the Pirates organization in 2003, coming off a solid year as a AA middle reliever. (He was one of five players the Pirates lost in the draft that year, including Chris Shelton and Jose Bautista. Rumor has it that this was motivated by money, that the Pirates actually were so poor/cheap that the draft price of these players made a difference.) He wasn’t ready yet, put up a 4.79 ERA, and it was back to the minors, a year of development lost. After 2005, he needed Tommy John surgery, and was cut loose. The Braves picked him up and he did good work as a swingman for Richmond. He made two good starts and one bad relief appearance for the big club; however, he’s much more likely to start the season as a reliever.
As a minor leaguer, Bennett hasn’t really been anything special. Even last year, he struck out just 45 in 86 2/3 in AAA, while walking 34; that won’t do. On the other hand, he was just coming off TJ surgery, and looked good in his major league work. It wouldn’t be unprecedented for a pitcher to have several valuable years at this age after general mediocrity, but it’s not really common.