Why Boycott MSN?
Danny Sullivan has noted the Microsoft move into the search crawling business in his article Microsoft's MSN Search To Build Crawler-Based Search Engine
Google currently has a massive share of the search market, and nobody has suggested that Googlebot should be banned on the grounds that google is uncompetitive, or that Google has a history of buying up smaller companies, technologies or archives to achieve market dominance - despite the acquisition of Blogger and the conversion of the Usenet archives into Google Groups.
I don't buy the concept that Microsoft are inherently bad and that any other web or technology based company are inherently good, regardless of their market share, simply because they are not Microsoft.
And if nothing else, it seems self-defeating. It looks like Microsoft are intending to build a web search tool into their next OS based upon what MSNBot crawls. If you are not willing to be listed there, I'm pretty sure your competitors will.