In practice, the IncludePlugin system was much better than an iframe: the “included” tiddlers were available for all linking (including freelinks), transclusion, search, etc. The only interface difference was that the remote tiddlers allowed only a “view” button where internal tiddlers allowed “edit”.
In other words, for the web visitor (who couldn’t edit anyway), there was literally no difference. The presence of “included” tiddlers was seamless – as if they were in the loaded tiddlywiki.
Yet from the authoring vantage-point, there was a significant benefit of having a “write once, read many” way of handling a central store of useful tiddlers. It was ideal for someone who had distinct tiddlywiki projects served up for different audiences (or oneself in different project-contexts), but still some need for overlapping resources.
In my current workflow, by contrast, if I tinker with important tiddlers in one file, I have to keep track of a workflow for propagating that change to others, and it’s easy to lose time in diff comparisons if I can’t immediately jump on opening the other files and dragging newly-improved tiddlers across.
Tiddlyspot hosted all of my TiddlyWiki projects at that time. If there was anything important role played by node.js in making “IncludePlugin” work, it was nothing that affected the end-user.
I’d love to hear from @simon or @saqimtiaz or anyone else familiar with how the TWC nitty-gritty differs from TW5 (or of course @jeremyruston — but only if nobody else can answer readily), as to whether this “include” possibility is definitively foreclosed now. (Freelinks did eventually make it to TW5, and so this difference is the only bit of nostalgia I still carry for TWC!)