I don't know about any other Pokemon fans, but I buy games with the same title because I want the same gameplay--not just for Pokemon, but for Mega Man, Castlevania, Kirby, Fire Emblem, Advance Wars, Tales, Dragon Quest, and most any series I like. All I really want are new skills and maps to play around with for turn-based games, and new skills and levels to play around with for real-time games. That's more than enough "difference" to me.
That said, with Pokemon I'm definitely in for multiplayer while the main single-player quest is just kind of there to me; on its own it would be something like a C or C+ RPG to me, not so much because it's simple (I don't mind simple) but because it's way too easy. Still, since I buy every version I manage to play through each region at least three times (except Johto, which I only played through twice) and don't really mind, since the journey is great for helping me become at least a little familiar with the new Pokemon and their types and abilities, and works as a collectathon for all the TMs and items I could want for multiplayer. And the Battle Factory in Emerald and Platinum is something I can legitimately pour hundreds of hours into on its own, though so far it's only in those two games.
