seeing as you can create far better weapons I never saw any reason to get the sigil
I am 100% certain that people get the celestial weapons fully levelled up for the bragging rights, not because there's no way to get stronger weapons than them.
Sorry, but one thing, in the postgame at least, is undebatable: the celestial weapons are doubtlessly the best weapons in the game. Why? Because they make your physical attacks completely ignore the enemy's Defense stat. This makes a huge difference against some enemies and makes customized weapons far inferior to the celestials in those particular fights.

Take Penance, the ultimate superboss, for example. His Defense is so high that at 255 Strength you'll still only deal around 20000 damage per hit with a customized weapon. Celestial Weapons consistently do 99999, even with a far lower Strength stat. In the end, they will vastly outperform the alternatives.

I find it kinda sad that they have that property, it makes customizing feel a little pointless. I'd have preferred it if they had just been regular Break Damage Limit weapons with a few free slots that you can customize at will.