Now you're talking about a malicious attack. If that's what it takes for this to be a real issue then I don't think many people need to be too concerned.
The cost of building a quality website & promoting it can easily hit deep into 6 or 7 figures.
The cost of throwing scummy links at a website is under $100.
And if their first $5, $10, or $20 test doesn't work? Then so what for them. They put it on cron job and throw another $20 at it again and again.
http://www.seobook.com/images/sape-sape-sape.gif
The key point here is that there's a 100X or 1000X gap between 4 figures & 6 to 7 figures. The cost of burning things to a crisp is much lower than building them up. Whenever incentive structures have a 1000-fold difference in outcomes the path of least resistance becomes a popular path.
