Miz isn't credible at the moment, he's been buried in every match he's in. He jobbed to Truth and Kingston cleanly, not to mention Cena. Had a decent run in the EC match and Rumble but that's it. He's good enough to become credible just by speaking though, that's why I'm putting him on SmackDown. It's about creating new feuds, have to remember that. Who wants to see Miz vs Cena/Punk/ADR again on RAW?
Del Rio is credible in kayfabe terms yeah and he's someone I've forgotten to put on SD, I'd probably put him there in the mid-card to boost the IC title, or put him in a tag-team perhaps with Christian. Both credible enough to boost those divisions.
Henry has been buried for the last 3 months, hasn't won cleanly for ages. Ziggler had a good spell but now he's stuck in a tag-team and isn't really needed on SmackDown, plus there'd be too many heels if I put him there before.
Jericho is pre-occupied and isn't around long enough for SmackDown, but you're right he's semi-credible. Swagger is nothing, is he even a heel anymore? #Sarcasm you get the point. Same goes for Kane. There's a difference between heels and credible heels who can main event without it being stale.
As said above, Del Rio is credible but can he get a reaction? Who has he not feuded with in the RAW main event? He needs to go to SmackDown in my opinion or drop to the US Championship on RAW. There is no credible heels on RAW, hence the move for Barrett and Rhodes.