Dark Age of Camelot had concentration-based buffs. If you want to limit the number of characters someone can buff, just code that. Recast timers are annoying; Bob wants to keep these five buffs live all the time, done.
Yes, it encourages dual-boxing. Yes, there are times when you do want things to be timer-based. But if Bob can keep this buff on five characters continuously, just let him do it, rather than having him re-buff every four minutes.