The whole MvP being FFA is something I don't agree with, but that's not what this thread is for and I will refrain from rambling about that.
I think Castle DB/BB rooms would be cool. It would also prevent drama ('cause no one likes drama) by letting guilds have their parties in peace and enjoy them instead of worrying about other players coming in uninvited. It's not that it's wrong, per se, but it kinda crashes the excitement if you have to move the whole group to a different location because someone else is in there being pest-like.
Hypotheically speaking, of course. I've never been to a DB/BB party, I just hear things about them.
Regarding the difference between BB-spawned MvPs and normal field spawn, if you look at it a certain way it's kinda like loot stealing.
But only if you use Offical Kaoreii Approved Twisted Logic. (TM, pat. pend)