The supposed advantage of different sites (I think) is that you can game the system to rank quickly for niche terms.
So, let's say I have a product that targets people searching for bodybuilding for the elderly (hypothetical, lol). I could register and quickly create a microsite at www.bodybuildingfortheelderly.com. Which is different, say, than claiming a unique brand name, say jubilados.com, and then pointing people to the bodybuilding for the elderly section on that site. I imagine doing the former would be quicker in the short term, and that doing the latter would be better long term.
Thoughts?