Were you using LinkWithin to link to exchange links on other sites? Or to link to related content on your own site? I think (?) it's capable of both.
Given one of the above comments, it sounds like LinkWithin runs the links through it's own server (with a redirect) so if you're mainly interested in interlinking your own content, I wouldn't use it.
Rather, I would using something like YARPP (it's a wordpress plugin; google it) to handle your related posts. There's no reason you can't do both related posts *and* focus on list building. Just have a nice, splashy box for subscriptions and then put the related posts after that.
Getting people to click around is only a part of the goal with a plugin like this. The other is to build connectivity b/t posts on related topics. And because YARPP doesn't redirect things through some outside server, you get the full SEO benefit of interlinking your articles.
YARPP is quite powerful, allowing you to tweak the threshold (and how it's calculated) for consider something a match, allowing you to allow/disallow specific categories/tag, etc. You can even check a box to insert the related posts into your RSS feed at the end of each post — this can be good for drawing people to your site if you publish a full feed (and you really should be publishing a full feed).
If you pop over to my site (link in footer below) you'll see that I follow each post with a prominent (albeit non-splashy) call to subscribe via e-mail and put the related posts a bit further down the page (between the meta links [categories, tags] and the comments). Again, while enticing people to click around is nice, I'm more interested in the interlinking.
Note that you won't always have great matches if you don't have much content in your archives (this is a new site, so you may only see 0-2 matches per article so far, and that's with the threshold pretty low) but it improves quickly. I typically allow up to 8 related article links, but that's also customizable.
Hope this helps.