@AndyC: Your TV analogy falls down because with a TV, no one is ever working with multiple channels simultaneously. On a PC however, I frequently am using numerous applications simultaneously (often upwards of ten). It therefore makes sense to have a conveniently grouped area where I can see those applications that are relevant to me right now, and a way to remove them from this area when I finish using any one of them. The taskbar does this beautifully, the Metro thing on the left does not because it has too little space and is not always visible.
Yup. Also the "initialization" time of switching between channels is not too bad. I got a quad-core and spinning up some processes until they allow interactive input can take seconds.
Channels don't have impossible to predict behaviors either that take random amounts of resources from the TV at any given time. Nor are they really stateful in any important way.
Also if you let the OS randomly ("intelligently") kill processes you can run into problem where the process was doing something important and now it gets stuck in a state where the next time it runs it has to read from a journal, or possibly lose state important to the user. That's assuming it is well designed and can even properly recover from invalid states.
Relevantly, this process killing thing has been in Linux since forever. However, it only runs when the system is under extreme duress in regards to available memory. I suppose you can have it run periodically, but that would be silly.