I never heard the theory that leaving it long-and matted keeps out the weed seeds. One other benefit to "short in the fall" is when the wind blows, the leaves keep moving!
I hear ya about keeping the leaves moving, can't disagree with that!

But, there is some goodness to those leaves if you make use of them... Granted, leaving a thick layer of leaves on your lawn over the winter isn't a good thing, it leads to rot and fungus problems and nobody wants that.
My personal choice is to let the leaves fall and then on a good dry day, mulch them into the yard. Set the deck about a .5" higher than the grass and run over them a couple of times to reduce them to small particles. Lots of nutrients in them and I'm all about turning that back into the soil for next year's growth. It's free fertilizer! :thumbsup:
Granted, Wikipedia ain't the end all, be all. But, there's a lot of good iinfo in there. Take a look at this
link, especially the last sentence in the "Water" sub-section:
"After the seedling emerges from the seed coat and starts growing roots and leaves, the seedling's food reserves are typically exhausted; at this point photosynthesis provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light."
If the weed seed germinates on the matted grass, it can't get any nutrients from the soil because it's not touching the soil. It will eventually die off...
I would think cutting or grazing short is kinda the same ... the following is pasted from the Noble Foundation ...
This goes with what I was saying about leaving it long. I cut my field grass at 3.5 on my Toro, and 4.0 for my bermuda in the civilized portion of the lawn. What you said just backs up what I was saying up post...