I played for about 12 years before I ever planned a team. I honestly had no idea for quite some time that people actually did plan teams, and for a long time after that, I still had no idea why they did. Since I've tried doing it, I can see some of the attraction, but it's still not my preferred way to play.
The first time through a particular region, I always catch everything I've never used before and use them long enough to get them to their final evolutions and get a good idea of what they're like. On later playthroughs, I pass up stuff that I'm sure I won't use, but I still catch pretty much anything that seems like it might even be marginally useful, and generally work out of a pool of up to a few dozen different pokemon, rearranging my team constantly.
It wasn't until I had already played and replayed White a couple of times that I decided to try a planned out playthrough. It was sort of fun, since I didn't have to sweat catching stuff or grinding stuff, but I thought it was a bit dull to just use the same six pokemon all the way through. Since then, I've done one in Black 2 and I tried one in Platinum and in Emerald, but neither lasted - I ended up rearranging my team anyway. I've got a vague plan to do a hacked one in White 2 with a "dream team," but I haven't been able to decide on a team yet.
My preference is still for just catching and using whatever I'm in the mood to use and letting my final team sort itself out.