If the Main Gauche attacks weren't normalized 1.5 speed axes etc. would have next to no advantage in damage per hit. With normalisation an axe would act as a 2.4 speed weapon, while a dagger would be a 1.7 speed one. This clearly benefits axes more than just having it act as 1.5 while the dagger was 1.4.
Main Gauche procs from main hand attacks only, so the proc rate would not be affected. However, the extra combat potency (and poison) procs would heavily outweigh the damage loss from 0.1 speed in a non-normalized situation, where the result isn't as clear in a normalized one with decent mastery numbers.
During WotLK going from 1.5 speed offhand to a 1.4 was often worth more damage than getting a full tier higher 1.5 speed weapon.
