If you're not going to Mega Evolve, then simply don't use the stone. There's no drawback to Mega Evolution when you're using it, so there's no reason not to Mega Evolve.
Um, I just explained several drawbacks with mega evolution in my post, you even quoted the drawbacks in the post you're responding to me in. And (to repeat myself again) you're right, there's no drawbacks for most of them, and you're right, you can simply opt not to use them, HOWEVER, as I said (again and again) the choice is taken away from you, for some pokemon. It's not as simple as choosing not to use it, for some you are required to EITHER train, build move sets and gets natures for it's original form OR it's mega form, you don't get the choice of using it or not using it as the decision has been made for you, you have to do one or the other.
Beedrill loses it's (potential) sniper ability and gets the (arguably) not as good adaptability so if you built a move set to capitalize on Sniper, well, too bad.
Pidgeot goes from a physical to a special attacker,
Steelix you lose something useful like Sturdy, Sheer Force or even Rock Head for SAND FORCE? Come on!
Sharpedo you lose Speed Boost for Strong Jaw. Not a huge deal, but you now need to focus on more biting moves unless you just simply don't want to make use of it's ability at all, but again, changing move sets based around the mega ability OR the normal ability
Altaria, Glalie and Salamence, while all good boosts, all require a normal type move in order to make use of their ability, (see above)
Diancie (who if you didn't know about the mega evolution like I didn't until half way through) could potentially have soft reset for a Brave, Quiet, Relaxed or Sassy nature, then it's mega stone comes and boom, it loses both defenses for an increase in speed and both attacks, again, totally nature and training situation if you didn't know beforehand, and you can't breed for another one to get the correct nature for the mega form.
Blastoise, again, COMPLETELY different move sets based on it's mega ability vs it's normal form. (except for one, it's pointless to have the pulse and aura moves in normal form, there's so many better options. And if you DON'T have the pulse and aura moves, then it's mega form is totally wasted)
Heracross, you must have Arm Thrust, Fury Attack, Pin Missile or Bullet Seed on it in order to make use of it's ability, which is fine, except that if you don't mega evolve, all those moves suck and there are far better moves for it, but if you DO mega evolve, you need to have at least one of those on it, once again, letting it's mega evolution dictate it's moves.
Garchomp, gets slower AND loses Rough Skin for Sand Force
All the rest where their stats get a boost in areas they're already strong in or they get their weaknesses bolstered, yes, you're right, it is a good thing, but there are those that you have to completely alter your strategy for a TEMPORARY status forcing you to, (to repeat myself again) ALWAYS mega evolve those particular pokemon or NEVER mega evolve those particular pokemon because everything about them hinges on normal or mega, not both.
