Why did Celestia think releasing Discord and reforming him was a good idea? Ever since he's been "reformed" he's caused nothing but trouble, and the ponies are clearly unable to keep him in line. It honestly seems that if Discord wasn't so disproportionately popular with the fans he would've stayed a statue.

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...well, better to have him on your side than against you. As Tirek showed, that battle would have been over in seconds had he not opened his mouth.

And to be fair, I think Discord feeling like was out of place was partially on the ponies as aside from Fluttershy up to that point they treated him as a nuisance.

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Discord is Chaos incarnate. Chaos can be contained for a time, but only with difficulty, and when he gets out he's so pent up with suppressed chaos he very nearly has to mess with everypony on the planet to catch up.

He's a force of nature, and the problem with stopping a force of nature is the same problem as making a dam with no outlet. Eventually, you are going to have a flood. The proper way is to make sure there is an outlet, preferably a safe one. Little pranks are much preferable to him taking over everything.

But the most important reason is that chaos is power. If you can harness that power towards the side of good, there is no end to the possibilities. Now, some of those possibilities won't be what you expect, that's the nature of the beast, but better to be a friend to the tornado than its enemy.

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Out of universe? It's because he's popular with the fans. That's about it.

In universe? In theory, at least, he could deal with any potential villain. As a reformed character, he does still cause endless trouble, but it's small-potatoes trouble. It's probably worth the inconvenience to have him on call.

It was a disastrous move for the show but especially for Celestia's character:
You should try to reform Discord because he could be a powerful ally. That's almost straight out of Darth Vader's mouth, and is a variant of "Friendship Is Power," it completely undermines the core message of the show. And Celestia had ZERO interest in befriending him herself, so it even adds an additional layer of foisting this on the M6 as, "Do as I say, not as I do." She put Twilight on the path of friendship to prevent another Sunset Shimmer, not just so Twilight could free Luna.

It failed to justify its stated purpose. After "reformation" Discord was an actual creator of conflict and net liability far more often than an asset, he expressly created suffering for certain characters and enjoyed the suffering it caused them. The overall effect is, Celestia looks more like Tyrantlestia, and really bad at it. After reformation this guy was a second away from murdering(permanently banishing to another hostile dimension) someone he'd just met because he thought he'd been snubbed.

Even supposing this guy is "friends" with one of 6, we're supposed to overlook his outright hostility to the other five and innocent bystanders too? Just because he's powerful? And interactions with Fluttershy are now poisoned because they necessitate interacting with a shitbag by proxy. It guaranteed going to Fluttershy's cottage becoming a source of dread for the others where it wasn't before. And all this is what we're expecting kids to understand? This is considered a desirable friendship? I wouldn't need any enemies with a friend like Discord. Or Fluttershy, because of proximity. Necessary Evil isn't a concept most children will understand, they aren't yet developed enough to process gray morality. Even then it wouldn't be fair to classify Discord as gray, he's too sadistic and a multiple instance betrayer.

Much like for most of the show writers' calls, I have no idea.

7082714 Well, Celestia probably knew that she'd just have to keep having him turned to stone everytime the Elements of Harmony passed to a new generation of bearers. Far better to get Discord on your side than to keep having him break free and cause trouble until he's put away again. Plus, at least initially he likely wanted him to help track down Tirek after realizing Tirek had escaped from Tartarus.

I always figured Celestia realized turning villains to stone wasn't really a solution to the problem, and didn't want it to be a never-ending cycle. Then the finale happened.

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I actually wrote a story about this back in the day, and I think it still holds true after all this time:

https://www.fimfiction.net/story/93137/war-games-of-the-mind

7082714 Exactly! That's the only reason he was brought back, because he was popular with fans. And in-universe, Celestia has an extremely bad habit of not thinking things thtough, which is why most of her plans blow up in her face and come back to bite her in the ass. Seriously, I could write an entire book on all of her failures as a ruler.