In this game i've been playing(m2tw), my early army is composed of militiamen with 150 man units armed solely with spears or bows, only.
By this I mean, I have five 150 man units with spears, and nine 150 man units with bows.
To this end I thought it was odd that no man in those spear units carried a sword or axe just incase, but nope just spears.
Was this an actual common practice during the medieval period or just a part of the limitations of a games programming?
I asked a similar question before and got some great answers.
I recommend you look at that thread to answer your question. Basically, from the response, uniformity wasn't very important, so units would be composed of troops with weapons that occupy similar roles. The weapons wouldn't all be the same, but often similar enough in use that they could be implemented as a unit.
Disclaimer: As others such as WARitter have pointed out, this answer does not cover the entire range of the medieval period, and is focused primarily on England from approximately the 14th and into the 15th centuries. It is a fantastic response from a very knowledgeable user, but is not representative of the period as a whole, nor of the various other regions of Europe.