Why do we need BizSpark? While we would like for our customers to use our web app exclusively, the reality is the majority of practices run Windows and Office. So we built Exchange and Outlook integration using the tools provided by BizSpark. If we hadn't had access to those tools we still would have built the product, but we wouldn't have Exchange and Outlook support at launch. The benifit for us; We built a product better suited for our customers. The benefit for MS: They have another product on the list of thousands that has custom support and integration with their office suite, their bread and butter.
Clearly the biggest benefit for us has been access to thousands in software we otherwise couldn't afford. But we will be looking at the opportunities to network with partners and investors through BizSpark when we're ready.
I see it as a win for us and an ever so tiny win (because of our small user base) for Microsoft.