The point of the whole "mega-pokemon" series is to add another variable in competitive play.
If a pokemon has to hold a mega-stone to mega-evolve, then it can't hold any other item. Think how vulnerable that makes a mega-blaziken. No sash, no swords dance setup. All you can really possibly do is give it protect and get a speed boost. It just allows for more vulnerable sweepers in OU and Uber-tiers, and makes use of those NU pokemon while making them look cool and giving them a different ability/type.