One of our reasons, though, is that we believe it encourages imagination and creativity. Toys with too specific of a purpose, or very strong associations can be very limiting - you only play with it in the way you are "supposed to," and that doesn't really require much thought. The more simple and basic the toy, the more things you can do with it, the more creative you're going to be with it.
We've seen lots of evidence this seems to be working with Guthrie lately. This past summer, we took him to see the Disney movie Cars. While he does have a little tiny actual toy model of both Lightning McQueen and Doc Hudson, he loses them a lot, and is really much more interested in the models he's found in his own giant box of cars (thanks Mitch!). He does ask for the licensed stuff when we see it in the stores, but he doesn't go overboard, and it's not a battle at all.
What he has done is find, in the box of cars he already had, models that he has assigned to be the cars from the movie. It started with a blue race car that actually did look amazingly like The King. Then he found other race cars, and one became Lightning, and another - a green one - Chick Hicks. His model VW bus, when not being called "Uncle Mitchell's Bus," is Fillmore. A green car with flames is Ramone, an older silver car is Doc. His red Coca-Cola truck is Mack, and my personal favorite, the tow-truck we bought him in Mexico, is Tow-Mater. There is also a black Porsche that has been assigned to be Sally, although I think Eric helped him figure that one out. And the big plastic car is Luigi. My favorite, though, without a doubt is his "gas station." It's part of his train set, and is supposed to be, I assume, a platform for waiting for the trains. I never would have thought to make it a gas station, but that's exactly what it is, and what he did.
Unfortunately, for some reason, I can't get all the pics to load into this post. So, see the post above, or below, or however it ends up showing on the page, to get it all figured out.
Ramone and Doc: