I'm getting this from the book "Fighting Techniques of the Oriental World".

In a passage referring to the battle of Kawanakajima in 1561, the authors state that "the spear troops of the ashigaru at this time were used in a slightly different manner to the equivalent pikemen of Western warfare. Although the spearmen would present a united front in defence, their spears forming a vicious barrier to penetration by horseman, on the attack the lines could be much more fluid and individualistic, each soldier charging with his own intent".

From this and the modern lineages of yari techniques from different koryu bujutsu, I believe that we can infer that the common ashigaru were taught the techniques of how to use the yari, but only placed in formation and left to fight on their own in battle.