The closest I can picture the Yahoo/MS battle to is two kids fighting in primary school over a candy bar. It always makes me smile.
That said, there's this rather big group of people who just think the world will come to an end if Google doesn't get competition, and that the Yahoo/MS deal needs to be done otherwise the world will come to an end. No kidding.
Truth is that competition keeps commerce healthy. Enterprises always try to be better than the other. They have to keep releasing new and attractive products, updating their current products, keep their prices down, and so on. A lack of competition allows the enterprise to go by its current products, their current prices or even higher ones, and above all it allows it into a monopoly situation which prevents competition and enters a vicious circle which is not comfortable for anyone but the enterprise itself.

Google has plenty of competition in a lot of areas. Yes, even the websearch market. Yahoo is, to Google, a competitor in websearch and, to a lesser extend, webmail. I won't go over them since that's not the subject. My point is that there is competition and competition.
Microsoft has been, since years, holding a monopoly position on a few markets, notably the OS and Office application market and, until recently, on the browser market. To get a common misconception settled right now: a monopoly is not a 100% market share. A monopoly is, in a few words, a situation where an entity can force a product onto its users without having to worry about serious sufferings.
If you don't believe that, go back in time a few years. Netscape used to have almost 100% market share. Microsoft released Internet Explorer and, surprisingly, Internet Explorer had that number all for itself just months later. While this post is not here to accuse Microsoft of monopoly either, truth is that Microsoft really is not the best choice of a possible competition. They were anticompetitive in the past and they still will be until Ballmer is fired.

But, this is Internet. People love to jump on the bandwagon and get back this bit of coolfactor they might have always missed while younger. Competition is good as long as it is real competition, folks. Some enterprises have a very different definition of it.