I've always thought that he kept it till Lavos for battle's sake, but that point about the Masamune is quite noteworthy. And Zephira, I've often wondered that myself: how would Frog battle in human form? No slurp, no slurp cut, and I'd assume decreased agility and mobility; having grown so accustomed to fighting as a Frog, he'd be severely hampered by the transformation.
In one of the main ending variation, Frog bathes in the Denadoro waterfall to revert to his human form. Is it just for kicks or is there's something notable to it?
Hmmm, I can't exactly remember, how do you get this ending? What criteria do you have to fulfill to achieve it? I guess I'll have to go re-watch it...Anyway, Denadoro is a pretty mystical place, having housed the Masamune and a whole load of Mystics, and it's also got a certain historical element to it: Glenn became Frog there, maybe he had to return there to have the curse lifted?