From the sound of Persephone and some of the others, I should never shave mine off (though I do have a big bushy goaty as well). Good things:
Not having to shave it all the time (if I’m going bare, I can’t stand a partially hairy lip)
Not having to look at all of my ugly mug all the time.
Playing with it (I get to do the end of the mustache twist)
Trying different styles with mustache wax (though I hate it, I have tried making it curl up, etc.)
Main reason not to have one: that one hair that gets thicker and then groes the opposite way from all of the other hairs.
I grow facial hair very quickly – so much so that I go from smooth-as-a-baby clean-shaven in the morning to raspy sandpaper before dinner. My mustache (and chin whiskers) prevent me from gouging the hell out of my wife’s face. That, in itself, is worth the extra work of trimming the curly kamikazes and all the other trouble I wouldn’t have to go to if I just shaved my whole face instead of my cheeks and neck. I’ve periodically asked my wife if she wants me to shave; she always says, “No!” and rubs her lips with the memory of facial rugburn.
Same here. Mine has only been shaved once in my life. The only people who have seen me without it are the ones who knew me before puberty. The 'stache now belongs to my wife, and has since we first met.
I agree that it gives a man something socially acceptable to play with in public and adds the tickle factor, but noone has yet mentioned the fact that it also makes a great scent cache.