I have long been skeptical--OK, I have laughed mercilessly--of those who want to believe that spears were used like staves, that is, for striking actions as well as thrusting actions. For one thing, there's no clear evidence for that in foot combat in any Fechtbuch. For another, spears just aren't built for that.
There is a hint in one of the Talhoffer books that talks about a "schlag" or strike with a spear, but that word isn't as rigidly used as we tend to think of words being used today, and in the context I really think it just referring to a "strike" with the point, i.e., a thrust.
However, a recent post on Will McLean's blog opens a different possibility. In a quote from a description of a 16th-century barrier combat, the text says:
"with the but ende of the spere strake the Almaine that he staggared"
In more modern English this would read:
"[the] German struck [him] with the butt end of the spear [so] that he staggered."
Now I still reject a striking action with the business end of the spear: The fact is, as Will has shown, that spears were tapered toward the point, making them thinner there, so striking actions were almost certain to break the spear if any real force was used, and what good is a light stroke? A strike with the other end, however, which is the thicker end, obviously, might be a different matter. Having said that, we still must ask several questions. First, was the strike a swinging blow or a thrust? Don't assume the word "struck" in the text is definitive--we often see the word struck used for thrusts (e.g., in jousting descriptions).
Second, how relevant is 16th-century barrier combat to Kampffechten? Many sources (e.g., Anglo's "Renaissance Martial Arts") are quite clear about how artificial such combats were, with everything from artificial techniques to weapons pre-scored to break spectacularly. It might be that a stunning blow with the butt of a spear was useful in a "friendly" (Will would say "consensual" to distinguish it from a judicial combat) barrier fight but would have been laughed at by someone seeking to kill his opponent in Kampffechten. We don't know. And third, if this was a generally viable technique, why doesn't it show up in any Fechtbuch?
Having said all of that, there is one plate in a Fechtbuch that shows an attack with the butt of a lance; it's in the Roßfechten section of two of Jörg Wilhalm's books. Here is the technique from one of them:
I'm not sure how this relates, however. We know that the forward motion of a horse changes the effect of strikes (e.g., Dom Duarte's comments about using the motion to add force to a sword blow), and we have speculated that a pommel "thrust" with a sword in foot combat could be effective against someone even in a closed visor because of the percussive effect it would have. Thus, it's entirely possible that a "thrust" with the butt of the spear would be similarly effective in foot combat. And such an attack might have been useful, if you held the spear long your hands would be closer to the butt-end of the spear, so you would be able to use it better in close combat (where you couldn't use the point because you wouldn't have room).
So, once again, we have a fascinating bit of evidence that doesn't actually tell us anything definitive. Sigh. Still, sometimes by building up little tiny bits of apparently useless data we construct a good argument. No deep, carefully researched conclusion here, just food for thought.