Also, dual-specs are in the game...

Glyphs follow the spec you're currently using.
If you have elemental shammies who both pvp and pve, but have no interest in resto or enhancement specs.

You might want Thunderstorm glyph for PvE, you definitely do not want it for PvP.
Eventually the cost of two glyphs (Thunderstorm in PvE, whatever you replace it with in PvP), will exceed that of dual specs.