The relevant link argument is certainly a good one. To me its more of the reputation that site you are linking from has with Google, i.e How many incoming & outgoing links it has well as Google PR.

If I was going to optimize a site for "mobile phones" and I linked from a site about "gardening" as long as that site has low outbound links and a PR 3 I consider it a great link, but its was a site about "mobile phones" it would be even better.

If you had a relevant link from a site that had a PR 0 but lots of relevant content I would still go with the gardening site.

Not saying I am a expert but that's my opinion.