Ed Felten’s been thinking about the question. Here’s his analysis:

Last week Microsoft offered to buy Yahoo at a big premium over Yahoo’s current stock price; and Google complained vehemently that Microsoft’s purchase of Yahoo would reduce competition. There’s been tons of commentary about this. Here’s mine.

The first question to ask is why Microsoft made such a high offer for Yahoo. One possibility is that Microsoft thinks the market had drastically undervalued Yahoo, making it a good investment even at a big markup. This seems unlikely.

A more plausible theory is that Microsoft thinks Yahoo is a lot more valuable when combined with Microsoft than it would be on its own. Why might this be? There are two plausible theories.

The synergy theory says that combining Yahoo’s businesses with Microsoft’s businesses creates lots of extra value, that is that the whole is much more profitable than the parts would be separately.

The market structure theory says that Microsoft benefits from Yahoo’s presence in the market (as a counterweight to Google), that Microsoft worried that Yahoo’s market position was starting to slip, so Microsoft acted to prop up Yahoo by giving Yahoo credible access to capital and strong management. In this theory, Microsoft cares less (or not at all) about actually combining the businesses, and wants mostly to keep Google from capturing Yahoo’s market share.

My guess is that both theories have some merit — that Microsoft’s offer is both offensive (seeking synergies) and defensive (maintaining market structure).