Now wizards having a low enough Refresh as it is probably wouldn't be able to take this but it could be useful for focused practitioners and sorcerers as it would allow them to cast more spells before taking mental consequences as well as soak more backlash (For the mental version of course). As for "Armor" it would be able to reduce mental/social stress from mental/social effects, but it would be pretty broken if it allowed spell casters to not take the minimum 1 mental stress for casting a spell, so the armor wouldn't work against something like that (But would help with backlash and reduce the extra mental stress for channeling above your conviction).
The common opinion on the forums is that wizard's are powerful enough without reducing one of their only limitations. Consider that any submerged level wizard could pick this up. With mental armor like that the caster will throw around even more powerful spells just because they can, not just when it's important or dramatic.
On a secondary, less important side I also have a thematic issue with a focused practitioner who is better at slinging around magic than the wizards. A focused practitioner built specifically with this ability will consistently throw around better spells than a full wizard.
I really have no problem with someone wanting mental armor/more mental stress boxes unless it's in conjunction with spell casting abilities. It's just not appropriate for any game I have run or played. Have any of you ever really run out of mental stress/consequences to cast spells? I don't think I've ever been in that situation unless it was dramatically appropriate (I.E. it's the big bad and I'm throwing everything I've got at it, and then it still takes a few more shots from my trusty shotgun).
Anyway, I guess if this is an issue that you've run into many times or something then maybe something like this could work in your game, but it's not something I would implement.