I agree. The same argument must be made for Small size then. Casters benefit a lot from having Small size. With reduce person, that becomes better. Plus, reduce person can be at will with a major ability.Yes, it isn't hard to get 10 ft reach, but try to get more than that. With a large size character weidling a chain or lance, you have your reach.
But even just looking at 10 ft reach, look at what melee weapons get most use on optimized characters? Why hello there spiked chain! Why hello there Lance.
Reach>All on a melee battlefield control/tank character.