Sev, its not a nice comparison.
Look - Lets say fully blitzed I'm landing something like 550 front number. And have GHTF + PTHF + Wild Weapons = 69% glancing blow damage.
If I gain an additional 10% glancing blow damage - that's another 55 points of damage per glance on top of the 380 I was already doing. If I twitch, 20 hits = 20 glances = 1100 extra damage every 20 hits (55*20). Sound ok?
10% doublestrike - If I twitch, 20 hits procs 2 extra hits each of which also has a glance, and those extra hits have a 40% chance to crit (in my falchion wielding case). So, I'm doing, at minimum an extra 1100 + 380*2(glances) = 1860 extra damage. If they both crit, well its lightyears ahead of the 10% glancing blow damage.
That 10% doublestrike covers the glancing blow, and probably the 400 damage barb capstone - because a barb fully maxed out probably won't go over 160-170 melee power - which would only scale that 400 damage up to about 1000 on every vorpal. Between momentum swings, holy retributions, divine sacrifices, that 10% doublestrike is constantly churning out extra crits and glances that I'd never have.