This is probably the side of the fence that I fall on in terms of this issue. If I see that obtaining links from paid directories isn't only not hurting our major competitors, but actually seemingly helping them rank on the first page and outrank us for many keywords, then I am natually going to assume that those links are helping their rankings.
One of our major competitors outranks us for more than one of our main targetted keywords and, having checked their link profile again today, they clearly invest a lot in obtaining links from directories, not many of them are relevant, what you would call reputable on first look and one is French, not even being in the site's own language!
Furthermore, although they don't do it for every link they have on directory sites, they have many links that use keyword-specific anchor text. Now, this is something that I would hesitate doing as often as they do, rather I would use branded anchor text when unsure of the validity of a link as I would expect that to carry less of a penalty if that were to happen, but it's clearly not hurting their rankings, instead seeming to help them.
In any case, this is why I referred to the use of directory links as somewhat of a grey area - it's not completely clear as to whether it's a viable link-building option or not. Given what I've seen of our competitors, I have to come to the conclusion that it's worth the risk.