In Microsoft's defense, most vendors are late to the game with these patches. We can all thank Uncle Sam for this one imho.
Anyway I wanted to concur with my colleagues above. I've got a few clients happily on Exchange 2000 and one migrating off of Exchange 5.5 and even though the patches are done, you have to pay $4,000 to get them. At the very least, doesn't it make sense for Microsoft to release these Exchange specific patches for these products to keep their customers happy. I usually defend Microsoft to a lot of the Linux/Apple lovers out there but this is a great example where Microsoft could shine by giving these legacy version patches available for free.
Just my 2 cents.
Larry