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The general response you'll see in the SEOmoz Q&A is towards spending your time on your main site rather than creating lots of micro sites. Arguments for one site include only having to update/maintain one site instead of five the next time there's an update to the software you're using, and not having to do all of your link building times five (if you have four micro sites and your main site). With microsites, you're often splitting up the value of your sites into pieces, rather than having all of the value go to your one main site.