pvp: sub ffb glyph for mage armor glyph?

I was thinking about it, sometimes against certain teams I find myself spellstealing a lot (maybe I shouldn't) but I can run out of mana and subbing ffb glyph for mage armor glyph could fix that. Another side effect of that is that if I talent into frostfire orb unglyphed ffb would be a 70% snare. Has anyone done this? I might try it out but would love to hear from others that may have actually tried it...