Before, I think this wasn't allowed because the section of a curve was
built in multiple parts. Now, after some work last year, a whole slice
of a curve is built in one section, so we can easily support curves with
all geometry types, including the new custom profile option.

Note that this also allows "caps" like in the following image. They could
easily be disabled by checking for "Full" fill type if that was preferred though.

(Also addresses this right click select request: https://blender.community/c/rightclickselect/8Fdbbc/)