Mercy is neither unreasonable nor misplaced, but also not the most helpful criteria in fight stoppages:
The crowd booed because they thought referee Steve Mazzagatti stopped it early, but it was a merciful ending for Ben Rothwell, who was tossed pillar-to-pillar for nearly six minutes. Cain Velasquez slammed Rothwell throughout the first round and turned it up a notch in the first minute of the second. In the first minute of the second stanza, Velasquez pinned Rothwell against the cage. When Rothwell tried to stand up, Velasquez nailed him with five unanswered lefts. That's when Mazzagatti jumped in at 0:58 of the second round.
If the justification for stopping a fight is that the referee can use mercy, then sure, the fight should've been stopped. But that's an awfully slippery slope to be treading. MMA fights can often be long, brutal affairs particularly when one opponent is in a clear but protracted battle to finish a highly durable opponent. There can be any number of situations where having compassion for a fighter won't be out of bounds, but will lead to judgment calls made in haste. Candidly, my compassion in the more abstract sense is given to them before they even enter the cage. One should always be mindful of a fighter's condition, but allowing mercy to take such a prominent role in decision making turns far too many difficult but unfinished situations into early stoppages.
It's not even that Rothwell's defense was excellent as he stood. His occupied hands left his face exposed for flush shots. So while not textbook, it was still unequivocal. And that is what the stoppage hinges on. He was actively changing positions to better his circumstances in his personal defense. Clean and simple. And to the larger point: my compassion for Rothwell's terrible plight does not necessarily medically indicate Rothwell lacked the conscious decision making skills to protect himself.
And does anyone honestly believe "Big" John McCarthy would've stepped in at that particular moment? I'm betting no. Hate him or love him, I don't think it would've been unfortunate had McCarthy refereed that fight instead.
I have mercy for fighters and their conditions, but when competing I need to know they are medically able to compete. I also need to know they're able to make choices and act on them to improve their condition be they winning or losing. I already know they're suffering and hurt.