Unless Flores slumps severely, I'm going to be very pissed if he's replaced by Lo Duca or Estrada upon their return. I see no reason why he can't permanently claim the starting catcher job with this stint.
I'd prefer to see Flores and Nieves trade starts for the rest of the season. They're both good catchers and I think they'd really feed well off each other. As for the other 2...dump 'em.