This has been debated and argued many times and we can argue until the cows come home and we still will not have an answer.
Of course being a boxing fan, I will try and be as neutral as I possibly can.
In my opinion, I think we are actually talking about 2 different sports with 2 different sets of rules.
It's just like asking a Badminton champion to play in Wimbledon and a Tennis champ to play in the All England championship. It's basically comparing apples with durians, not even oranges, remember Mohd.Ali vs Antonio Inoki in a farcical contest held in Tokyo.
Well lately we have MMa stars trying their luck in boxing and they get thrashed by unranked boxers.
Mighty Mo, Ray Sefo among others come to mind.
Just a few months ago, boxing legend James " Lights out " Toney faces former UFC champion Randy Couture, Oh goodness what an embarassment it was to boxing fans, Randy only need 3 minutes and 19 seconds to choke James out, forcing him to surrender by tapping out.
I saw the fight and I feel Randy was being very merciful, he could have ground and pound James till his face become a pulp, but as ever, Randy is always a first class gentleman, in and out of the ring.
Actually a real thrashing would have done James a lot of good.
So my take is there will never be a clear cut answer to this question, maybe in time to come, some smart Alec can devise a so called in between rules and regulations that will be fair to both contestant.