Well if you develop an existing fan base then tell them later they will have to pay for the game, you will lose every one of them.
Are you referring to roguelikes specifically, or games in general? I thought I remember seeing some games that were free in alpha/beta stages and then transition to pay after the stable release, although I guess they announced the pay intention from the beginning.
I checked out the Legerdemain page and you're right it is a pretty cool model. Lot's of value added with collectibles, maps, giant book, etc.
To my knowledge, Legerdemain, Unreal World, and Dwarf Fortress are the only roguelikes (this definition may be stretching) that generate money for the developer. I don't think financial reward for roguelike developers is a bad thing though and if I ever can rich-ify myself in these coming years I would definitely support some of these cool projects out there.
Having said that, for a roguelike to generate money it has to be DEEP: deep gameplay, epic awesomeness, and something that sets it apart. It is interesting that there was a recent, sometime in the last couple of months, thread at tigsource where a poster asked if a roguelike could make money by selling the game and surprisingly many responses said, I don't see why not, but...