Wow... it's weird reading through this thread. I knew I was in a minority since I rotate through lots of pokemon through the course of a game, but I had no idea it was that much of a minority or that pronounced a difference.
I mean - just in my current B2 playthrough (which is my third time through Unova), I've been using Crobat, Growlithe, Ampharos, Servine, Leavanny, Lilligant, Trapinch, Magneton, Vaporeon, Azumarill, Scraggy, Lucario, Gothita, Sigilyph, Krokorok, Drilbur, Dunsparce, Lopunny, Darumaka... and more I can't remember right off. I've got Eevees lined up to evolve into Espeon and Umbreon, and another that I think is going to be a Leafeon and before I'm done, I'll catch some more for the other eeveelutions (I particularly want to give a Flareon a good try in-game). I've got a Volcarona in the box that's definitely going to see some action soon. And I'm about to go through Chargestone Cave, which means I'll at least add a Tynamo and a Ferroseed to the rotation, and probably a Joltik too. And I've still got a Minccino to work on and evolve, and a Deerling that I want to get into a summer Sawsbuck. And even without whatever other pokemon are in B2, I'll certainly pick up a Litwik, an Elgyem, a Pawniard, an Axew, a Cubchoo, a Cryogonal and a Stunfisk along the way. And probably a Palpitoad. And a Vullaby. And a Deino. And so on...
I honestly have no idea how anyone could NOT do that. How do you actually manage to see and at least have the opportunity to catch interesting pokemon and not use them? That's like getting a new car in a racing game and not driving it.
When I first read this question, I thought it was really kind of weird. I'd have to struggle just to limit myself to seven pokemon - that's just a fraction of what I normally use - and here it's being treated like some sort of complication to the game. But then reading through the responses, it appears that the way I look at and play the game is COMPLETELY different from anyone else.
I just never dreamed that that was the case. Again, I knew I was in something of a minority, but... wow. I had no idea...