Like others have said, it really doesn't matter to me whether or not the shirt has dates on the back or not. If I like a design, I'll buy it. It's the specific memories behind the shirt that matter to me - especially if they gain a bit of damage in the process (the pulled collar for my favorite Slayer shirt, the shirt Otep yanked out of my hands and signed without me asking, my bloody GWAR shirts, the Powerman 5000 shirt I wore to a Denny's that pissed off some old lady because it had the word "FUCK" on the back...).
Tour dates make it a lot easier to remember the date it happened along with the "I was there" statement - especially if you saw Iron Maiden - but in the end, they're just a few pieces of cloth stitched together with something printed on it. Plus, when you think about it, there's the thought that you hopefully gave the artist some gas money to entertain another group of people, the venue owners some money to keep a chunk of the local live music scene active (if merch fees work that way, fuck if I know), even if the venue is a total shithole, and allowed the band to know they still have people that want to buy their shit.
That's the way I see it anyway. Didn't mean to get a chunk of "stoner philosophy" in on this thread.