It is simply more efficient to send the relevant metadata in IRCv3 tags than adding the overhead of passing all that data as separate commands. The data is there when it’s needed. The batched data is unreliable (may not be up to date), so the other option would be the same as the old way, sending a chunk of extra commands before each PRIVMSG, which is rather wasteful. IRCv3 data lets Twitch add or remove any metadata from messages without having to create arbitrary commands for metadata and implementing all of these RFC commands, making sure that they comply with the standards etc.
Then, I’d prefer chat as a streaming JSON api anyway. No parser ambiguity, and when WebSocket is implemented, removes a ton of unnecessary work for the web client. But that’s just me.