Okay, Let me explain you the short story :-
After Google Updates on blog network, quality content, etc.., Many of them switched over to guest blogging for ever lasting result and not to affect with any of the further Google Updates. So, More people jumps into guest posting from blog network, even guest post cost higher. As the result, there were many guest posting service arises without proper knowledge & Google guidelines. I still wonder, how they can give quality guest post for such low prices..
Now, Google turns towards Guest posting it seems, but one who does guest posting with proper guidelines need not care about Updates.
As we all knows, Guest blogging is wonderful and it's adding value to the internet for the people that are doing it right. Just don't use a re-branded article network or blog network as guest post for the sense of making money. I suggest, the best strategy is to move guest blogging on lots of sites to being a regular "contributor" to a few select.