Ah... I was going to say the thing about soldering. I've had a few games (mostly belonging to other people) that I've transferred new batteries into , but I've never had a trouble with a pokemon game. Not even when my GBA got tossed in the wash with my jeans along with Ruby. xD
The only game I've ever had experience battery death was a mega old non-color copy of TLoZ: Link's Awakening, and that thing is epically old... Then again, I have a copy of Metroid 2 for the GB that's probably almost as old as I am, and that still works. I still play it from time to time. It's gone through a lot, it was my father's before I got it. Gamer family heirloom. xD
I think it all has to do with how kind you are to the game cart, not how much you play or how old it is. I bet more often than not the battery comes loose or corrodes or something rather than actually dying. Those 'flat batteries' are pretty hardy little things, aren't they?
And don't they get some kind of charge whenever you play the game? I'd think so, I'm sure Nintendo would see to that, they may as well since a circuit needs to be closed anyway, aye? Or maybe not, I'm no hardware expert, I just know how to put things back together after I pull 'em apart. xD