Are you honestly going to tell me that picking the scraps of other teams minor league systems will get you to produce a viable pitching staff? You're kidding right? All the guys in the low minors have warts, hence being left unprotected. As for live arms, go ask Sam Doduno.
Look at the Rule 5 history: Rule 5 draft results - Wikipedia, the free encyclopedia Recognize many people on this list? Me neither.
In theory you could build a rotation out of that. In theory I could conquer Asia with a spoon, but it ain't gonna happen.
FYI... RA Dickey was taken by Seattle in the 2008 Rule 5 draft. Worked really well for them too. He went 5-8, 5.21 ERA, 1.56 WHIP that year. Yep, you can really build em.