Yes I know that I may have to be slightly grey hat for this.
Here is the problem with guest posts. If I build relationship with people (blog owners), then they'd know more about me. They'd know where I work and what I do. The problem comes when doing a guest post for someone else. Most bloggers I know do not allow you to post guest post linking to your clients for the purpose of SEO. There are some that do, but majority of the A listers, don't do that.
So when we have to build link for clients, it's difficult to get links from them unless we have a very good say in how the blog links out. This is best achieved by private link networks.
Of course there are other ways. But my question revolves around this idea. How viable is it to build a network of sites (Domain sites or web 2.0)