If they're interesting products, I think that's cool. We're capitalists here:-) No spam, though, of course.
Affiliate links?
Why not, if it's an interesting product? I don't begrudge one of us making a bit of cash for showing us something cool. On the other hand, if someone's spamming, throw the book at them! It's pretty clear that 'frisco' is a user who participates in this site and is not a spammer, though.
I just don't really see the problem.
http://news.ycombinator.com/submitted?id=ravindra1982
http://news.ycombinator.com/submitted?id=insomniamedia
From a low wage country it doesn't take much to get someone to spend a few minutes / day to 'participate', and then to drop the 'payload'.
Anyway, it's sort of a moot point, as it's not really possible to automate elimination of those sorts of links.
There are plenty more these two are only a small sample.
I do wonder how much you earned the Stumbleupon user though. A quick google search on the affiliate suffix shows he links to a few other DX products on SU and elsewhere. Today I'm sure he saw a boost in profits.