Uh, there
is a big bill coming at the end. You need to keep your system running, even though you now have to pay for licenses, don't you?
When building your system during BizSpark, you are unlikely to make architectural choices with respect to software licenses, because they are free. Then you graduate, and you're probably looking at a system that uses way more licenses than it would have if you had to optimize around that parameter from the get go.
BizSpark is an excellent move by Microsoft, and likely is helpful for folks starting out who already know the .NET stack. But I'm very skeptical that working in the Microsoft stack offers such a technical advantage that it is worth the risk that your business is not going to be in a position to suddenly have its operational costs bloated by software licenses by some arbitrary date determined by Microsoft.