One of the most common responses tech savvy people make when they find out my job is to get websites higher up the search engine rankings is "Oh, so you have a big network of sites and link use them to promote clients sites".
My answer is and always will be a straight "No".
Most people still think of SEO as some kind of dark art where companies trick the search engines into giving them top rankings but this is becoming more and more inaccurate.
The problem with getting links from a network is that unless the network is full of high quality real websites it's pretty unlikely that Google is going to want them influencing the search results.
If you own 10 high quality blogs and drop client links in every so often then that isn't a problem.
The problem is when SEO companies own thousands of sites and use them heavily to link to client sites.
If you work with one of these companies then what happens when Google decides links from this particular network shouldn't count anymore?