While it might be more successful to have 8500 (fairly large) seed investments, Microsoft is not a venture capital firm. If one were an investor in MS, I'd would wonder why they were using my capital for other companies and why I shouldn't just invest in them myself?

Companies that have their own VC arms tend to be very focused on a vertical where synergy can exist (ie. Intel). For MS to make 8500 investments they'd have to spread their money so wide that there would be no focus in the portfolio.

Whereas with purchasing Skype, they can roll it into existing platforms and conceivable increase the value of it.