It still isn't that helpful anyway unless you get someone frozen and can separate yourself or you're up against another caster. This is because any melee that is whacking on you will be dancing all around you trying to make your spell misfire by being behind you.
The glyph can be nice for pvp though.
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On top of that you can't do much in 20 seconds.
That certainly isn't true though =D 20 seconds is an eternity in PvP for a Mage.