There are two metrics - rankings - here, and they're potentially in conflict.

Firstly, the position of a site in search results - i.e. how high up a site appears - can be affected by the number of sites that link to it. That's the effect that donotlink.it reduces. It's not the only factor that that affects a site search ranking.

Secondly, a site's 'page views' ranking - i.e. how many people actually go there - as measured by Alexa and others. This has a big effect on how much a site can charge advertisers. It also affects search ranking.

So the question is - if people use donotlink.it in their posts will it cause more people or fewer to visit the target site? Those who wouldn't normally click on a link to the Express, say, will end up there, increasing their page views and revenue (a tiny bit). And if they detest the site too that'll piss them off. People, like me, who would want a good reason for clicking an obfuscated link will not click it and will treat future posts by the same author with suspicion.

So if you use it, tell people where it goes.