10 Mind-blowing Microsites - Definition and Uses
If you look at from the Search point of view: it gives you a targeted destination for converting keywords, it provides more content associated with your brand that you control, and the sites may get you another place on the coveted first page of results. On the down side: many developers of microsites do not design them in a search friendly way, they don't tend to be linked into the rest of the world, and rarely do they have enough text-based content. If I were developing a microsite and would like it to be visible in search results, I would make sure it was search friendly.
Imedia Connection recently posted a really good article by Fred Brown on the subject. Read the Article on Microsites.