If some glyphs can be get into the PC part of community, while these glyphs are supposed to be rare, it would be crazy to allow every player get it. The glyphs simply stop being rare.
Think.
It doesn't matter that you was unlucky and feel that you should get it, because you are special. You are not, you are just another player and you can't have everythink. What matters is the PC community in general, not you.
So if there is time limit beatwen succesful attempts to finish a quest, its quite good solution.
Although, if someone use another character to finish the same quest multiple times, or does it with different alts only to sell the glyph, its big abuse.
It doesn't break the amont of the item in the PC community balance, but it is highly unfair for those of us who doesn't cheat and abuse.