Mega Gardevoir's physical bulk is somewhat mitigated by Will-O-Wisp, which beats Scizor and Aegislash. Mega Gardevoir has two main sets: Offensive and Calm Mind, both of which are strong, the Calm Mind one being a bit less important than offensive. Mega Gardevoir is a flawless Latios / Latias counter and has a highly spammable move, it doesn't need to be versatile. It can simply fire off its Hyper Voices well, not everything has to be extremely versatile. Mega Venusaur isn't very versatile and its in A Rank. And I honestly do not consider the "it takes up a mega slot" an argument anymore. There was a use a while back, I believe Subject 18, that made a long post about it, but to summarize: You use that Mega Pokemon because you want what role it plays. Mega Venusaur is never wanting to be switched out with Mega Pinsir on stall teams, simply because its mega brings what it does to its team. Mega Gardevoir brings a powerful special attacker with a highly spammable move, the only Mega I'd consider in its role is Mega Charizard Y, another special sweeper. While 100 Speed may not be the best, several things also have that speed or lower, such as M-Zard X and Aegislash, and even though it has poor physical bulk, you're ignoring its super large Special Defense, which is something it really has going for it. What support does it really need? The only I've found is a way to beat Scizor and Aegislash which Will-O-Wisp handies dandily, I'm sure there is more but that is right off the top of my head, feel free to correct me.