- Customer base / Traffic: This seems to be most plausible reason. Yahoo attracts more traffic than MSN and Microsoft might want to buy that. But then assimilating overlapping set of customer base very different technologies would be a very challenging task.
- Search Technology: Microsoft has been pouring millions to enhance its search engine capability and might be looking at Yahoo to better its search technology. After all, Yahoo is doing better than MSN today in terms of search capabilities.
- Middleware Synergy: This is least likely. Microsoft and Yahoo are on two very different technologies (Windows Vs Linux / BSD) and it would be very challenging task to switch the OS and middleware. Even assuming it is doable, what would be the business case for it?
Regardless of what happens to this deal, software industry is in for some big changes and customers are in for some beneficial but exciting ride