I agree with the notion that it should require continued kills to maintain because half the reason it becomes a problem in the first place is because the DPS getting it are usually are playing extremely conservatively, hanging back and only moving in when they're confident they aren't going to get picked off for one reason or another. IE, ranged DPS often won't approach at all if they see that there's a WAR or DRK in an opposing group who could yank them out of the group and get them killed. I also see NINs who basically do an opening Trick Attack, burn Bunshin, then run away with Shukuchi the moment their defensive cushion is gone.

Having a timer ticking would force people to take risks if they want to maintain the buff, which would create a more interesting dynamic on both ends.