Greg Benage
Legend
There is no other valid reason. "I don't like it so you can't have it" is a childish response not worthy of a DM.
In many cases, it's a very clear player signal that they're trying to power-game. Sometimes they'll just outright say so. "Why does your draconic sorcerer want to make a pact with a powerful fiend?" "So I can get eldritch blast, hex, Pact Magic slots, Agonizing Blast and Devil's Sight!"
That said, I allow MC, and I also have good players who use it to fill out their character concepts and not just to upgrade their at-will attacks or whatever. If I had a table full of optimizers, I'd allow it and never think twice. If I had a mixed table, I'd only be concerned about it if I thought some optimizers were angling to create a PC imbalance that would make the game less fun for others. I have enough experience as a DM to manage that, but some don't. Simply opting-out of this optional rule is an easy solution for them. The easy solution usually has costs, but it's certainly a rational choice.