Because I want the keywords to be in the URL, even if they are written in Japanes, Chinese or any other non-English keywords.
If I use ascii, this topic title: “どういたしまして どういたしまして”
becomes
/t/topic/17
I used it in all my websites, it’s great for usability and make the URL meaningful in search engines after indexing, and it’s useful for SEO as well.
You already have a percentage encoding option, so you just need to apply lowercasing, so it will work as ASCII for English letters with small case and will be compatible with non-ASCII characters. Wordpress and all the other popular blog and forum software have this.
Google also shows the URL with the decoded characters, not the encoded ones.