This is really not a big problem. A non-activated version of windows will run for a while before rebooting. It's mildly annoying to have to reboot every now and then in extended testing, but it works sufficiently well.
Microsoft's suggested solution is to just start from the raw image each time you start the vm. This way it will run without problems for 30 days. The images come pre-setup so you don't need setup time unless you want to install some extra debugging tools.
Why? They have no reason to pass WGA, and this is only mildly annoying for actual users, as a development VM I don't remember it ever causing me any issue.
[1] https://github.com/xdissent/ievms