Sometimes I will use it in a conversation when referring to a third party unknown to the person I am talking to and the subject's gender has no bearing (like in my first comment).

For example, "Oh, I know a guy that knows X," "I know a guy that is a fan of X" or in a less positive manner "That guy just cut me off [on the freeway]!" (since you rarely see who it might be and their gender doesn't matter)

I could say "dude" as well, but sometimes dude might be too informal. I could reword it with a number of other pronouns, but it depends on context and whom one is talking to.

"Hey, guys!" in reference to a mixed group is perfectly acceptable. "That guy" in reference to someone you do not know the gender of is also perfectly acceptable. Directly referring to a known female as "guy" is less so.

In phrases like those the word "dude" is more common than "guy", and it is frequently directed at women.