Problem is, the topic itself may be worthy of discussion -- but then you limit the people involved in the discussion (who have read the article) only to those who have a subscription to that given site.

What would be more useful is, where possible, post a non-paywall link, to maximize the number of potential participants in the discussion. An alternative in cases where the paywall link is the primary source, is to post a summary in a comment. But then you risk having the editorial slant of the summary be tilted towards whoever wrote up the summary.

I think it's maybe a little optimistic to assume that only people that read an article will brave a comment.

And even so, is it that big a problem if subscribers to some publication are the ones that can make an interesting comment about an article? Where's the downside to hosting that discussion?

An article has to be pretty special in quality for that level of exclusion to be worth using it instead of a similar article elsewhere.