from a practical point of view:
a good pin head is still better than a bad square head imo, but a good square head is better than a good pin head.

from a theoritical point of view, it is about higher Reynolds number and lower local lift coefficients, which leads to a less early stalling and hence more lift and less drag. Also the mast-to-sail-size ratio is more favorable if there is more chord. The gust unloading theory, which Rick explained so well, does probably the most good for the sail, especially for uni-riggs

Cheers,

Klaus