Lmao how the fuck is its only move worth using Explosion. Double Edge severely maims anything that doesn't 4 times resist it, Body Slam still stings and has a good chance to cripple very prominent threats that switch into Mega Glalie such as Gren and Keld, EQ has great coverage with Ice STAB, it gets STAB priority, it can stallbreak with its immense power and Taunt. Spike stacking is great for Hyper Offensive teams. Not only that, but you only explode when you don't need Mega Glalie anymore, like sacking a Pokemon for offensive momentum except it's so powerful it does OHKOs ANYTHING that doesn't resist it, and not even resists take it well. Do you even know how to play hyper offense? Sacking pokemon is inevitable against a good player and why not do a FUCK TON of damage while doing it.
It is slower and more frail than Mega Metagross, but Mega Glalie has power over it(Refrigirate base 120 Atk Double Edge hits harder than Tough Claws base 145 Atk Meteor Mash), it actually hits harder than every single one of those MEvos you listed(mLop, Megagross, and mPert). The only frail physical nuke to which you compare it is Mega Medicham which is equally as fast, even more frail, doesn't have the ability to set up entry hazards, and can't stallbreak with taunt all at the cost of power.
This is a goddamn horrendous argument as this goes both ways. When you "play around" it you're having the sheer potentiality that it might come in oppress your decision making, for which the opponent can "play around" and freely actualize his momentum to punish you and that could easily cost you the game in many scenarios.
So what if it's overall not as good as other megas? No one is saying it's a top-tier mon, but it's definitely not a bad option.