Despaigne probably sucks but really, so does Simon. He's my SP9 and on any half-decent team he's probably not any better than an SP7.
And Despaigne is at least young and pitching at Petco. Also, if you've ever seen him throw his repertoire is kind of interesting. Sinker/Cutter up to 94, curve, slower curve, harder slider, weird-grip changeup, and an eephus. He's still very much a "prospect", I think. Remember when the Blue Jays gave up on Henderson Alvarez because his stuff wasn't going to translate into strikeouts?
Rule 5 picks should have become tradeable as soon as the Rule 4 picks did. So yeah, you can trade them if you want.