Fulmination only uses a maximum of 6 lightning orbs. Which means if you have 5 orbs on you, it will use 2, to bring it back to 3. If you have 7, it will use 4.
The purpose of the glyph is to ensure that you consistently GAIN orbs by casting your spells, and you will no longer lose them if you have a rogue, feral druid, warrior or another melee class on you. This prevents said class from focusing you when they see you have 6 or 7 stacks on you (in order to prevent a massive burst unleash). No matter what, you will keep gaining orbs instead of losing them when someone attacks you. I'd say thats mandatory for elem PvP...
EDIT - Read it wrong. Seems useless.