Agreed, the helmet (should) be designed to withstand any expected bodily (capital) secretion (ew!), but over time of pulling the helmet on and off, certain bits will get compressed each time, and over time the helmet may become a less snug fit, and may become an inappropriate, and possibly unsafe fit. Real world? Maybe the degradation won't be severe enough for you to need to replace it before the recommended life of the helmet.
I've a 10 year old Caberg kicking about. I personally don't think it is fit for use any more due to age (it has had little use). However, it's technically not mine, so I can't freely dispose of it, although my father in law (the owner) is considering getting a bike and thinks he should just use that rather than get a new one. As I just mentioned in the camera position safety thread, a helmet MAY be more than capable of lasting for (say) 25 years, but the effects of ageing plastics and EPS protection is unknown, much like the effects of solvents and adhesives on helmets, so you should NOT use a helmet beyond the recommended time stated as the products used to make the helmet have not been tested for this time span.