Japanese here. This is my opinion:
- 【】 is commonly used for titles or categories (e.g. 【速報】(news)) in Japanese text. So I feel 【】for ruby is a bad idea. I don’t know why Japanese StackExchange chose it. Seriously I want to ask about this decision to StackOverflow’s developers.
- Aozora Bunko also has ruby’s syntax for plain texts. It is《》. (description and example.) This syntax is literally the biggest example of ruby syntax in the world. Originally, 《》 is used by the Books for the Blind Association in Japan. And Aozora Bunko follows it. If you say “let’s use【】 for ruby”, then I will also say “why not 《》? ;-)”. Aozora Bunko doesn’t adopt Markdown though.
- 【】and 《》 are hard to type for normal keyboards. It’s also a bit hard even with Japanese IME. Currently all of syntaxes for Markdown are composed of singlebyte characters and these are easy to type with English keyboard. I hope this defacto standard will be kept.
- I believe ruby is not only for japanese text. It is just a expression of all language’s text. Someday it may be used with a great idea for some language. I mean we don’t need to stick to specific language like Japanese.