From what I've noticed, the guy with the 17-odd sites has simply put up 1 original site and 16 semi-clones of the original. In other words, they do not seem particularly useful to web users, and instead simply seem to be more "crap" that has been thrown onto the web---though the 16 semi-clones are not technically duplicate content, but rather more like the dolly sheep; that is they are close but not exact. For his main keywords, this guy/company/whatever is having little success at having more than 1 of his sites rise to the top of his serps. But, on lesser competitive keywords, you will occasionally see page one and page two filled with his various sites. In this area, he's having substantially more sucess with yahoo and msn than google.
In my opinion, this constitutues spamming. Why? because these 16 semi-clones, while not being duplicate content, are "stealing the limelight" from other sites that might otherwise be found on page one and two of the serps and which might provide useful information to users. However, as we all know, many users will not go beyond page two of a search.
So, you would think that google might possibly frown upon such behavior. However, issues like this are seldom ever black and white or cut and dry. As I stated in my example, I could imagine coming across a content niche in which an individual had several sites that were generally devoted to the same topic and for which the content overlapped, but which also provided new and completely original content per each website.
I could further imagine that this could be wholly beneficial to users as long as each site contributed to user's desires. Example: you have a site about cars in general and you have five other sites, one devoted to custom car interiors, one devoted to aftermarket addons, one devoted to garage equipment (like cherrypickers), etc, etc. Let's say each of those sites had helpful tips sections on car repairs (all originally written for each site) and in the serps for "car repairs", you consistently found pages from each of these sites showing up in the serps...
In such a scenario, these multiple sites (offering similar but not duplicate content) might be very beneficial to users IN THAT they would be providing exactly the type of information that users are looking for. But there's also that other issue which includes the following questions:
1. is the guy with these multiple sites crowding out other sites? (answer: possibly yes).
2. is the guy with these multiple sites benefiting users? (answer: possibly yes).
3. is the guy with these multiple sites spamming the serps? (answer: possibly yes).
and the 64,000 question----->
4. does google view this as duplicate content? (answer: who the heck knows)
And this, of course, leads me back to the question with which I started (I think, getting hard to remember at this point after this very long post): does google regard multiple sites on the same topic as "duplicate content" or is duplicate content simply exactly that: d-u-p-l-i-c-a-t-e content.
I doubt any of us really know. Personally, I think webmasters (other than the ones who are clogging serps with MAF sites and pure crap) should simply put up their sites, not worry about it, and let google sort it out.