The neo-Fidelius spell is unpredictable. Sometimes it kills you outright (Dumbledore), sometimes it just robs you of your magic. Sometimes you get it back (Frank), sometimes you don't (George).

I think with Frank we may have done a random determination (dead or just magic-deprived? permanently or temporarily?). The others may have been done because it would make for a really good story. (We did not want to kill ONE TWIN like somehow that's less bad than killing any of the other Weasleys, but one of them paying a profound price along the way seemed like something we needed.)

(Does this answer sound right to the rest of you? Please chime in with corrections/alterations.)