Terry Semel at Yahoo! was useless. Steve Ballmer didn't sink Microsoft, so he had some value.
That's a bit too much dislike toward Steve Ballmer. The enterprise market is completely owned by MSFT today (huge part due to him). Azure wouldn't be Azure if all the enterprise products/contracts already in place. Sure, they missed an opportunity on mobile, but so did BlackBerry.
The stock market (stock price) just didn't love MSFT despite strong financials during Ballmer's tenure.