Just an impression from reading other posts, a wether and doe pair will sometimes lead to undesirable results within the herd dynamics depending on the personalities involved. Even though a wether can't impregnant a doe, he may still be able to detect the heat cycles and could engage in annoying the female(s) with chasing and mounting behavior. Also, a wether can be a rough and tumble male when it comes to dominance over a female and naturally has an inherent desire to prove he is the "top goat" within the herd. There could be times when the wether would head butt, ram, and purposely try to intimidate the female(s), and possibly excessively so during certain times. Male goats will test their strength, sparing abilities, hierarchy and breeding rights with other males. If other males are not a part of the herd, the female(s) seem to take the brunt of the male behaviorism. Female goats also have a hierarchy within their herd and can/will display signs of dominance against each other very similar to what could be seen between males.
All this being said, a wether makes a wonderful pet hands down. The dynamics within a herd of males is much calmer than a mixed herd, and the males will test each other to establish the hierarchy. Male goats are better equipped to endure the tests of strength, ect., between each other "man to man" without the worry of potential harm to a female herd mate(s).