Discredits the UFC as a real sports organization, and just a company who puts people beating each other up.
It's seeing if bigger and stronger is better than smaller and quicker. That's not what you want in a real sports organization. You want to put 2 guys who are similar in terms of physical specimen, and see how skill plays out.
Not to mention, it takes away all structure and ranking. If you have Alexander Gustaffson beat up GSP, does that make Gustaffson a #1 contender, or one of the best p4p?
If there was a fight fans REALLY wanted to see between 2 weight classes (a-la GSP/Silva), they already have a solution - catch-weight fights. Though it still kinda throws off the structure, the outcome of the fight would still be relevant.
In conclusion; in terms of idea and possibilities, it's pretty cool. In terms of reality, I personally never wanna see it.