I have started with the principal that I wanted to put all my efforts into one site because it is better to have a single site at no.1 for key search terms rather than 20 sites sitting on the 5th page.
Nearly 1 year on and I am glad I used that philosophy it seems to finally be paying off, however I have recently purchased several domains that contain specific search terms with the intention of having several small sites/blogs to use to promote my primary site.
I think the comment "diluted efforts" sums it up in 2 words.
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Alexcruncha
FantasyBingo.co.uk
CasinoReviewSquad.com