@E-123
The show has had a bad habit, which they finally broke in S9, of making Princesses dumb and useless. This choice was made in S3, where this problem was in full swing.
Celestia’s
idea was understandable: Discord is the strongest individual entity full stop.
If he could be corraled, his power was a weapon of equal value to the Elements of Harmony, if not even greater.
And indeed, it’s later basically bluntly said that if the Three Amigos hadn’t bell’d him, he would have been able to take them all on by himself. After all,
he was able to resurrect Sombra and forcibly teleport the four to his location
even from within Tartarus. Pinkie Pie, using whatever fraction of his power she was able to wrangle, was the immediate attack that won the war.
Celestia’s mistake: She did an Eggman, and tried to control a power that could not be controlled so whimsically. Even with “good intentions”, Discord turned out to be more harm than good both in petty life and in context of the fate of Equestria. The
only force keeping him on their side was his boner for Fluttershy. He didn’t even care about any of the others, frequently dismissing gross endangerment of them until Fluttershy specifically was mentioned.