If you nerf this, Mages as Support will be a lot less enticing.

TL;DR Spellthief's Edge allows mages to do more damage as a support than as a laner before first back, and even then Frostfang lets them match that if not still put them ahead.

5 AP is kind of meh, but 25% base mana regen helps fuel the likes of Brand to cast more spells than they would in the mid lane (moreso if they took Meditation), and the bonus damage isn't too shabby either. The real kicker is that they get +10 damage on spell hit. (And 8 Gold but.)

Compare this to a midlaner: they start with Doran's Ring or Dark Seal, both of which give +15 AP. That means, given the choice of trading with Support Brand or Mid lane Brand, who does more damage?

If you guessed "Support Brand", you're absolutely correct. Support Brand does more damage because he doesn't have 100% AP scaling and is better served by 10 additional damage. That 10 AP translates to 6 magic damage on his W, and 0.15% mHP damage on his Passive.

Support Brand, meanwhile, does 30 extra damage.

Then he gets this bad boy: And Brand does 15 more damage and gets 10 additional AP, for 500g. What can a Mid laner get for that money? or , for a maximum of 20 AP.

The score is: Mid Lane Brand: 35 AP Support Brand: 15 AP, +45 damage on spell hit.

Does the 20 AP compensate for the +45 damage? Only once Frostfang's exhausted, and at a 60% AP scaling, you'll need to land four spells to deal 45 damage, whereas Support Brand only needs to hit three times, whether with spells or AA's (and as I recall each tick of his Passive counts as a separate instance? Shit's crazy, yo).

Now, Mages going Support hasn't been raised as a Significant Issue, but I feel nerfing the bonus damage on Frostfang will pull back the reigns on Support Mages by a fair margin, since they'll finally do less damage than if they were mid lane.

EDIT: I notice the flaw in my argument in that mid lane Brand won't back and buy just a Doran's or Amp Tome. You're absolutely correct. But for a direct comparison between items, I felt this was more accurate. Mid Brand would probably purchase at the time Support Brand buys Frostfang, at which point he needs three spells to hit for 45 damage.

Mind that I neglected this argument because only his W has a 60% AP scaling, his Q and especially E have a lot less.