He not only loses unnecessary speed, but he also wastes a mega slot that could be used. Not only that, but he loses his signature ability, Rough Skin, which is a large part of why people use him in the first place. On top of all that, when he mega evolves, he has to hold a mega stone, which makes him unable to hold any items like Rocky Helmet, Dragonium Z, Choice Scarf, and Groundinium Z just to name a few. In all, the only circumstances where Garchomp-Mega is better then regular Garchomp is on a sand team.
Summary: Having Garchomp as a mega wastes
- A mega slot
- An item
- Rough Skin
- Higher attack
- Sand Rush
Yeah, seems pretty obvious to me Garchomp-Mega sucks.
Side Notes: A Pokemon's design has nothing to do with its usage stats so you mentioning that in your description of the question doesn't mean anything.