There is no need to differentiate "kind" if you are using regular web hosting. However, there may be cases where you want to host only HTML for exploration or share only JavaScript for code sharing. But in the current relay implementation, it is not possible to restrict inserts based on the value of the "m" tag. Therefore, to implement relays like the ones mentioned earlier, you would need to either develop custom middleware or modify the relay. Hostr aims to provide diversity without altering existing implementations as much as possible, which is why this specification is in place.
Discussion
So, if I understood correctly, the issue is that current relays don't use the MIME type to filter the events so you end up with notes you don't want in your page right?
Then wouldn't a single dedicated "kind" be enough to let it filter just those and then use the said "m" tag for the MIME types?
I think that's true for clients. However, what I am talking about is filtering from the author to the relay side.