It's more pervasive than just registration too if you allow the username to be adjusted. This is again a problem with email addresses that also allows leakage.
Regarding the probability of attack, people should monitor the number of different usernames attempted by a session/IP not just failed attempts against individual accounts. Otherwise it is very easy to try thousands of username combinations with a selected weak password.
The code to do this is on Github (at least for the UK): https://github.com/fubralimited/php-oara Affiliate network integration is not a technical challenge at all.
You'd be surprised at the level of data the affiliate networks make available to the people using them. Even if you are a small affiliate, purchase/basket level data is on offer.
Until the ICO start taking people to court it will stay ridiculous, but I think the intent of it is important. Using etags to track individuals is really no different to a cookie with a unique ID...
Having a 10 page complaint form is what is dumb. Why can't you just send a suspect URL, then have it scoured for dubious cookies?