It is worth it because there is no way to customize weapons that are as powerful as your celestial weapons, not even close. Sure you can copy all of their customizable benefits but there is no way to imbue a regular weapon with the "ultima weapon" effect, where the strength of each hit depends on the condition of your character's HP. Trust me, I've done thorough investigation and experimentation. The celestial weapons are undoubtedly the best weapons in the game for each character.

Plus, getting the sigils and crests really aren't very difficult, you can get them all in a day easily. Re-writing the spheregrid to eliminate +1, +2, +3, +200, etc. That takes dedication.