Main Gauche:
Your Main Hand attacks have a X% chance to grant you an extra Off Hand attack. Each point of Mastery increases the chance by an additional Y%.
It seems to me that, to benefit the most from Combat's Mastery, the Rogue would have to use a FAST MH weapon with a SLOW OH weapon.
The reason behind this is that a FAST MH weapon will strike more often, meaning the X% proc will get a chance to happen more often.
Also a SLOW OH weapon will strike harder when Main Gauche does proc, resulting in more DPS from Mastery if using a SLOW OH.
Now as far as i know, Rogues are supposed to use a SLOW MH weapon and a FAST OH weapon to achieve maximum DPS.
Which is totally opposite of what Combat's Main Gauche needs to perform as well as it can.
So, what is the catch here?
Has Blizzard really fu*ked up Combat's Mastery that much?
EDIT:
Personally i would suggest changing it to a "Proc Per Minute" system, and make Mastery increase the Proc's damage.