Shouldn't it be four variations? (Or maybe only two if you count a tick and a tock as the same.) Would it really kill providers to offer something like Nehalem-1C-4GB and SNB-1C-8GB?
Making major purchases once a year and always getting the new stuff, you get 4 processors over the life of the hardware warranty (4 years, which I also believe is the timeframe for capital depreciation).
Purchasing 2x a year, you'll end up with 8 CPUs in the environment unless you opt for the 2H purchase to be the same as the 1H purchase.
If you're buying new hardware every quarter, those agreements become more important if you want to keep your environment homogenous.
If you're running thousands of servers, is it better to pay a premium on your hardware to be able to get the same machines for the entire year, or do you want to incremental improvements and environment fragmentation resulting from updating? Different models sometimes have different utilities to manage them, and now you're writing abstraction layers to handle mass updates.
I agree that predictability of performance requires dedicated hardware. I'm just going from the IT perspective.