"Um, what? There's so much more running than what's being shown on the Taskbar that your reasoning is off by at an order of magnitude. As someone who's started pinning more items to the Taskbar about a year ago I can easily say that it hasn't affected how I know what's running at all."
It has nothing to do with knowing what every program running is, what I mean, is if I have a program pinned to the task bar, in a rush, I sometimes think it is running when it is not but should be, or if I have a program running (with it's icon in the task bar), I think it is just pinned and not running when it is running but should not be. Besides, the task bar is not a good place to put a lot of items, and even if you love the idea so much, then continue using it in Windows 8 as you would in Windows 7. The comparison is really between the start menu and the start screen, because everything else is the same except the start screen is a more efficient replacement for the start menu.
"Personally, I like a clean Desktop, and I can't believe that I'm going to be arguing this point. Your reasoning would be correct, if having a Desktop full of icons somehow prevented more windows being open or reduced performance in some way. But having a "messy" Desktop doesn't interfere with running any program in any way shape or form. The reason why a Desktop covered in icons isn't great is because it isn't an efficient way of launching applications. It creates a jarring experience to minimize/close all running programs to start a new program and then get "jarred" again by bringing all of the windows up again."
There are multiple reasons, personally I would find that having a bunch of short cuts on the start screen and trying to copy-n-paste, delete, and manipulate the temp files I put there would be irksome. What you put forth, is just another reason it's not the best idea.