You forget the bit where the "preview" is just that, a preview. How long before it expires? Then what do I use? (This is precisely what we do btw)

Relying on Microsoft's release cycle to get access to a "preview" OS is not exactly a sound testing strategy. If MS is serious about startups supporting IE (and therefore windows) they need to provide the tools to allow us to do it without having to buy a PC and a Windows license JUST for testing.