- If you get the layout, including both how it should look and how things should work, from an external source, or if there is substantial external input, use templates. It will save time and headache.
- If you are just about the only one responsible for the layout but support skins (for lack of a better word; the CSS part), use the HTML tag functions and macros embedded in Arc.
Since Arc is at this point targeted for exploratory programming, the former is an unlikely case.
What I would like to see, though, is a templating engine akin to HTML::Template  in Perl 5. It has the minimum of control structures and extra syntax on top of standard HTML, just enough to make it possible to use the template for dynamic content. All templating frameworks suffer from the same fault, though: they define a new but severely restricted embedded language that's mixed in an unpleasant way with the HTML source. The language embedded in Arc is a much cleaner way to do things.