Best interest of shareholders or short term speculators? which one?
I am always surprised when I see the phrase "best interest of shareholders" hijacked by short term speculators who don't care about the long term well being of the companies they "invest" in. Do they really invest in the company? The word "invest" shares a stem with the word "vested", it means you are entering into a vested interest. Speculators do not actually do that. They only invest in a short gain scenario, they sell out too soon to be considered to have a vested interest in the company at any point in time. They have an interest, yes, but not a vested interest.
If I look at the large mergers of the last decade, most of them have turned out to be disasters for the long term shareholders. They only ones to benefit from such mega-mergers or takeovers were the speculators, not the long term shareholders. Daimler-Benz lost 10 billion dollars on their adventure swallowing up Chrysler. For that amount of money they could have paid off all the speculators and still fare better as the burden of the merger would not have dragged them down. In the end they decided to get rid of Chrysler again. HP and Compaq is another such story. The combined value of the new company is less than what HP was worth before the merger. HP would likely be more profitable if they hadn't swallowed Compaq. Compaq itself was only up for grabs because they had swallowed DEC before and were unable to digest it.
So, who is to say that a MSFT-YAHOO merger would be a success in the long term. Speculators would make a killing that's for sure, but chances are that long term shareholders would lose out over the next 5-10 years. The IT industry is littered with mergers/acquisitions that destroyed the business of the acquired company and did not produce the expected and promised outcome for the acquiring company. At the same time, the consumer is worse off every time there is one competitor less. Ultimately those mergers erode the competitive landscape and that is not only bad for consumers but also for the competitors themselves.
For these reasons I seriously call into question whether Yahoo's refusal to merge with Microsoft really was against the interest of shareholders. It certainly was not in the interest of speculators. Other than that, this can only be satisfactorily judged a number of years down the road.