Let's look at PBF. Belts stop mattering when he won them all and they stopped when he didn't need them. Cotto hasn't gotten to that level, most fighters haven't, so belts mean something to them.
Storylines are meant, in real sports, to augment the fight not be the sole reason for the fight. That's the difference between boxing/MMA and pro wrestling (other than the predetermined outcomes, of course).
A, arguably in this case, compelling story that doesn't lead to a meaningful fight is a waste of time, something fighters don't have much of to begin with.
That would be a compelling fight with real consequences. Cotto-Margarito II isn't even an eliminator (unless Cotto loses). Horrible, indefensible fight.