If you put a cooldown on MR the little PVP that is left (limestone pvp or at other spots) will stop. 1 guild might have MR on cooldown while the other one just used it, making it impossible to bash eachothers heads in. Same goes for rpking, someone in your guild buffed MR when you were offline, you log in and you cant pvp/rpk for the next few hours because you don't have MR and you can't rebuff it.
The main reason people play is simply to bash other players heads in withouth turning red in the first 3 minutes of your killing spree. Change this and most of the veteran community that plays for PVP (those are the best geared and most $ spending players) will quit since half of the times they log, they can't do sh1t coz MR is on cooldown.
What you will have left is a very small community that only farms AFK on hk, which is probably not enough to sustain the server from generating enough $ to keep running.
Like I said before, I have played for 10 years and I've never quit (took a few breaks inbetween tho but always with the intend to return), not when they killed my beloved fishing maple xbow bug, not when epics came out, not when they released more upgs, server merges and what not, I played through it all. But as soon as Origin decided they wanted to cap the amount of PK/use of MR I quit that server and I will do exactly the same in Jainus, together with A LOT of others if they decide to change MR. I'm not playing this game to sit around and wait till you can pk for 2 hours and then wait again, I play to smack ass whenever I want, whereever I want and as many times a day as I want!