Brush up on your dictionary:
http://en.wikipedia.org/wiki/Fallacy
I'm not saying it's a large effect, or even a good idea, I'm saying that it is not fallacious to suggest that the effect exists, as it very clearly does. My original post was looking for some reasons, so I thank you for passing that along to me, though I could have done with a good deal less snark. I will say for my own part that I have experienced the benefits (and sometimes detriments) of deck thinning via fetchlands. I'm very familiar with the mathematics involved. But, you know, it's funny, somehow I've noticed that I tend to draw fewer lands when there are fewer lands in the deck....
It's usually small edges that decide games played at high levels. I'd rather give myself every edge that I possibly can. Is that a ridiculous position?