Think about it this way. $1.2B in cash would purchase ~39.1 shares of MSFT stock, had they repurchased stock instead of buying Yammer (at today's open of $30.70/sh, per yahoo finance). Those shares have a proportional claim on $106.7M of after tax earnings over the last 12 mos (approx $2.73/sh, TTM, again per yahoo finance). $1.2B represents 1/250th of MSFT's market cap.
Even if Yammer grows revenues at 100% y/y for the next 10 years (it won't), its cumulative after-tax earnings won't come close to the benefit shareholders would have received--esp when discounted appropriately for the risk--had MSFT simply bought back $1.2B worth of stock.
Don't get me wrong, Skype represented even more egregious profligacy. But my point is that I (and many others) would be far more interested in owning MSFT stock if they fired the entire corp dev team and replaced them with a small laptop that merely issued orders to buy back stock every quarter.
Silicon Valley used to view MSFT as a rapacious shark. Now it sees MSFT as a well-heeled but dim tourist--an interloper that has more money than brains and is willing to spend exorbitant sums for shinky trinkets. Sadly, MSFT has warmed to the role.