We used to have discussion subpages in the era when we didn't have talk pages. What is now the nature of a discussion subpage? I stated on Robert's WME that WME and /discussion do not go together, but realizing we have talk pages, I'm not so sure anymore.
I find that both are useful. When discussing go diagrams, I find it very useful to be able to edit someones diagram to add letters, markers or lines of play. In such cases, keeping a lot of stuff on one diagram, with comments appearing next to that diagram, discussion pages are more useful.
In cases where the discussion does not revolve around go, and does not require that you be able to edit what other people contributed, the talk pages are very useful. Especially since you don't have to link to them, as they are all grouped and available under a single link at the top of the page.
WME of discussion can be useful, but I think the result should generally not de named /Discussion, because then future discussions are going to interfere with the WME. Better to name them something like /DiscussionOnTopic? or /TheoryBehindTopic?, etc. With 'Topic' replaced with the appropriate topic.