I'm guilty of using it!
Simply because, if i've got fast blast type of ride out, if it's wet, i'll give it a miss, yep, that might make me a fair weather biker in a way. I see it more as saving myself for a better day! I have ridden 365 days a year in the past, so no newbie to riding in the wet, or ice and snow.
If it's meet up and ride out like whitby last weekend, no problem, still went, and it was peeing down when i left home.
If however, i was out purely to get leant over and through corners as fast as possible, then i'd save it for another day if it was wet.
Just my own personal preferences
So does it have a standard definition for me? Nope! Lol, depends on the situation and type of riding planned