One thing that people often comment to me is if I have a spare helmet that they can use to go riding with.
The answer to that is 'no', I'm not in the habit of keeping helmets around for no reason, but is it even a practical consideration? I mean let's say some random person asked to ride and you said 'yes' for whatever reason. For some reason I'm just not seeing them fitting into any helmet I might have in my collection. They're just too customizable and personal to have a 'generic' one. Heck, my helmet took forever to figure out the fit.
Or am I wrong, is there a helmet that generally works for - say 'normal human female'? If so I don't see any reason not to get one. I just kind of assumed that if you bought a helmet it was for that person, not everyone who wanted to ride with you.