On reflection… this makes more sense if I think of the allowed base types as things which can take bindings — something which is important for GET, SET, THE to work sensibly. That is to say, you can add sigils to words, and to things that contain words. This feels reasonable enough to me, even if it gives you a bunch of useless types.
It still doesn’t explain why you can’t add sigils to things like THE-WORD!, though. I still maintain that that capability would be useful to have, considering that the various sigils have more or less orthogonal semantics. It also doesn’t address my second bullet point (that Ren-C has this elaborate structure but doesn’t seem to get much use out of it).