If you want to 100% complete Kingdom Hearts you have to get everything (dalmations, trinities, keyblades, spells, etc.) and I think you even get a prize for doing it. But in Halloween Town there is a trinity that is in oogies mansion that you can never get after oogies mansion is destroyed, never again, so remember to get it as soon as you walk in (it's a red trinity mark and it is right near the entrance of where you walk into the mansion).