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I'd add the critical factor is "will this micro site stand on its own?" Meaning - is there enough unique content, value and trust for this site that it deserves to rank for its own phrases? If the micro sites share most or all keywords or content as the main site, or if the quality of the micro site is poor, or if it is only filler content purely for liinking, there really is no difference between micro sites and a link network. So if you want to go the route Charles describes (and I too have used with great success many times for many clients), be willing to achieve those goals.  If you think you can "get away" with artificially faking it, you may get away with it for a while.  However even if you do, all it takes is one manual review or an algorithm update to wipe out all of that work, and even possibly flag your site in a way that would require four times as much effort to recover.

Maurizio's got it.