The proc chance wouldn't matter much in most cases... But an increase to glancing blow damage? Definetely would and should have an impact if the devs cared about balancing the capstones in any way.
Having good glancing blows IS important to me, and im sure to allot of players.
How much an increase is some pretty basic math and not a big amount:
Say you do 100 base damage per swing, and 40 glancing blow.. Fairly typical maxed out endgame figures.
Over 4 normal attacks with GTHF (3 glancing blows)=
400 main damage, 120 glancing blow, 520 total dmg. If we wanted a 10% increase of that, we'd need 52 more damage, so 52/3 = 18 more damage per glancing blow
So mainhand 100, glancing blows 58
= 400 main, 174 glancing
= 574 total, 10% increase
18-20 more damage on glancing blows would be fair and really what should have happened. Could even round up to 25 to account for the fact they can't crit.
When a good programmer opens up a bugged file for a feat/enahncement/etc.. He would fix it entirely and for once to match it's description and be balanced. Not partially nerf it, leave i still broken and try to claim it was a bug fix.
That's the sloppiest coding I've ever seen.