It would have been annoying, yet a pretty big communication/skillful use of the ability if you could change them during an encounter. Right now because encounters are short and the disparity between retribution and the top classes (at least at 100) is fairly large, in practice BoM can account for more then 9% of damage. Not surprising, as there is a post slightly above that details that. If you could actually swap it around it would be a constant micro management of putting it on people when their major CDs come up, and since different classes have different CD lengths (typically in minute intervals) you would probably be destined to at the very least find 3 empty GCDs before every minute to line up with the next set of burst CDs. Is that interesting? Yeah, but is it fun? Apparently not. I do wish that BoM didn't fall off people when they fucking changed talents, I have no idea why as it really doesn't break anything.
I have to ask the question though, why would you not use 3 BoM in a raid? Blessing of wisdom is terrible, and tank deaths are usually the tank (or in this expansion a combination of tank and healer) fucking up. Kings is alright, but in no world would you be using it in raids. 100% if you're brought you will be using 3 BoM. PvP will obviously be different though and is about the only place (at least in 3s) where you will see the other blessings used.
In my honest opinion the best thing they could do for retribution is not balance them around 3 BoM and just up their damage to compensate. Making the blessings more powerful with individual CDs (2-3 mins) that you can throw out whenever you choose during combat would make it infinitely more useful and interesting. Or just still not balance around 3 BoM and make the others more attractive to where you might want to use them in a raid.