In SEO and link building establishing the value of a link is one of the biggest challenges out there. All indicators have advantages as well as drawbacks, so in theory using a combination of metrics is the best idea to implement across link building activities.
There is also a significant difference between bad links and weak links. The so-called weak links won’t impact on rankings as much as better ones but won’t harm your site, whereas links from suspicious neighbourhoods might attract a penalty and are generally to be avoided in order to keep your link profile healthy in SEO terms.
Even if you use a combination of indicators, such as PR and Raven, the outcome of your analysis shouldn’t be regarded as set in stone, since even Raven scores are subject to change or sometimes the way scores are established in the first place is revised. As a recent example, Raven is now using Majestic indicators as part of its score, and that will affect scores across all types of sites.
I guess in the end links are valued based on the ‘feel’ of the evaluator, which is a result of quantitative and qualitative factors. This leads on to the discussion that SEO is not an exact science, but an art, hence it can be approached in a variety of ways.
In the end, a combination of indicators plus a qualitative assessment based on ‘common SEO sense’ should be both effective and practical when it comes to decision making on link quality.