"A fight might finish at 5:01"
Then why not make it a six minute round? Wait, then it might end at 6:01... But then again, if it's a ten minute round, what about a fight ending at 10:01? Why not just go to 15 minute rounds like PRIDE did in it's early events? Time limit is arbitrary no matter what, unless you want to go on until someone gets finished, which would be a complete disaster. Three rounds of five minutes is plenty of time for two conditioned fighters to keep up a good pace and have a good chance of ending decisively. The majority of fights already end in the first or second round in the UFC, and decisions are usually fairly straightforward.
More rounds won't change matters, fights will still go the distance, just the fighters who are already having a hard time with a 15 or 25 minute fight will have to slow their pace if they hope to finish a fight that's suddenly ten minutes longer. Boxers go 12 rounds now and have to wait for a decision, when the fights were 15 rounds, they still went 15 rounds and got a decision, before that, boxers would go for more than twenty rounds and still eventually go to a decision. Decisions are a part of every combat sport, because a finish for every fight within a set time limit just isn't feasible. It's time MMA fans see the writing's on the wall and accept the simple and obvious fact.