Sahil Malik has launched on the air this idea some days ago and I'm totally agree with you about it: Beta or CTP Products could be distributed via VPC Images (downloadable from MS site for example) and we'll be ready soon to test the products, without troubles on installation (that occours often when installing different Betas or CTP).
Now I see a rumours launched by Scott Galloway: maybe next week there will be a publicly (well, MSDN subscriber) avaialble download of a Virtual PC image of Visual Studio Team System 2005 (plus all the bits you need to run it) avaialble soon. Wow, sounds good! 
Maybe Microsoft is listening this idea and will launch soon a new way to retrieve products for testing? I hope so... 