Blimey, what alot of vitriol about a (fairly) simple question!
Since the answer seems to be YES it is legal, I will leave it at that. I don't like being on SBY after a duty, but them's the breaks, as long as it is 'legal' then I have no problem with that.
To answer another point, "is it to cover for roster instability?" Oh yes indeed, the usual story of not enough crew or a/c to cover the roster properly. In that vein, what is a standby crew? ours are always flying!!
Actually, Stan hit the nail on the head with his first post........