What I plan to do for our next cruise, maybe in Aqua on a Japan cruise, is get a quote from TA before getting on board, then get a quote while on the ship, which we have never done. And compare which has the better price and which has better OBC. If not a big difference, I could see a benefit to keeping the booking with Celebrity.
I could see us checking out other cruise lines if they include OBC, maybe Princess? I don't see us doing Regent or Oceania or Azamara right now as even though some of them are all included, we don't always book suites.