in a way

i think relays rejecting notes that are non-valid according to the spec with a clear message would make for a better developer (and eventually user) experience, and not be censorship: you can post any content as long as it's correctly formatted, at least things are not accepted then silently hidden

however, what it would trade away is flexibility; that relays have knowledge of the message kinds and do checking makes it harder to experiment with new extensions and kinds, and possibly get in the way of "other stuff" uses, i definitely get the point why it was designed like this