I keep seeing people saying how Mega Charizard is going to be great because it has Drought. I thought about it a bit, and this is what I've come up with.

First off:
>M-mega evolutions have priority!
We have no reason to think this. Every piece of literature on the subject so far suggests that you megaevolve as you attack. Meaning AS YOU ATTACK. I'm going to assume that this is the case.
>You only have to attack once to stay Mega Evolved for the whole battle!
Yeah, I know that. There are some issues with getting that first attack in.
>Just give it Protect
Charizard doesn't learn Protect.

To briefly cover the whole thing, Mega Charizard has its uses, even on sun teams, but not as a replacement for Ninetales. Mega Charizard will be as brief a fad as GyaraVire when it comes to filling the role of Ninetales.

A lot of people seem to only think about Mega Charizard only in terms of how it will perform against other weather teams. After all, for any other weather ability user, if the other team isn't weather, your weather will definitely be up for the whole game, so it's not worth thinking about, right? Mega Charizard is different. If Charizard dies before getting its weather up, your sun team has no chance of getting off the ground and you lose.
In order for your weather to come up, your base 100 speed, effectively itemless Charizard needs to get a turn in. With 78/78/85 defenses, there's a lot that can kill you. Any attacker with base 120+ offensive stat using a move like Brave Bird is a guaranteed OHKO with neutral stab. Fast attackers which carry a water, rock, or electric move can also OHKO easily. The best example of this is with Lead Aerodactyl. If you choose to lead with Charizard, you'll not only be forced to switch immediately without getting your weather up or risk fainting, but you'll most likely have to deal with Stealth Rocks before you try to come back in later.