Yeah, I do it for a number of reasons. There are some really great recipes out there that have been made into kits. Good beer is good beer, whether I create the recipe or not. I'm not willing to discount great recipes out of pride.
By brewing the kits that I buy from various suppliers, I get to try out their recipe exactly the way they created it. When I use a recipe from a book, I don't know the exact grain supplier that was used. I can use Briess, Rahr, Great Western, Weyermann, Muntons, Gambrinus, or Dingemans. It's fun to experiment and change recipes, but sometimes it's fun just to leave things the way they are.
I don't currently buy bulk grains. I would like to, but I don't right now. Buying kits means that I don't have to store anything. Even when I build my own recipes I buy the exact grain bill, or as close to it as I can given the minimum purchasable specialty grain weight.
Sometimes kits come in cool looking boxes, and it's just fun to open them