the advantage in blogs (autoblogs in particular) is the massive amount of content you can get very quickly. Traditionally the main goal of blogfarms is to gain a large network of sites with many pages indexed, and use them to power your other sites (either via 303 redirect, or link power).
Another reason splogs get massive amounts of traffic is trackbacks. If you have hundreds or thousands of posts on the blog, there is a hell of a lot of trackbacks, which indefinitely send you massive traffic. added to the fact that it's very easy to set these things up, its a sweet method.
It can be either short or long term, depending on your goals and methods. sometimes (especially if you have a dedicated server), it's better to set these things up so they post hundreds of posts/day and simply shut them down after a few weeks and setup a new batch on new domains. So lets say you create a few hundred of these (very easy if you have a system) every few weeks, as you cycle out the old ones. If you are doing it properly, and making them post a few hundred posts a day, they should easily pay for their own domains, plus extra within those 2-4 weeks. Plus they should be getting a nice (or very nice) amount of traffic, mainly from trackbacks (if you're using RSS feeds). When you shut them down, you can post up any simple page with your keywords, and some ads, but delete the blog. The traffic can continue coming in for a long time.
Doing this, you quickly amass a large amount of free aged domains on a multitude of different niches. So if at some point in the future you decide to create a "dog training" site, you just grab all the dog, and dog training related splogs, and 303 redirect them, or link out, to the money site. Or better yet, have some intermediate "gray-hate" site, so that your BH splogs aren't directly linking to your WH or money making site.
And continue building a fucking money machine
I'm sure some of you recognize this method from Eli's amazing blog (Bluehatseo.com) from which I've learned a lot, and recommend to everyone. But for anyone who doesn't read it, I hope this helped a bit.