This is an interesting point - my first thought was isn't this solved with paragraphs, that can obviously be variable lengths? But what about favourites and retweets - I disagree with you that it's easier to read parts as separate tweets than paragraphs would be, but the UI to retweet or favourite a paragraph seems awkward and I think those functions on parts of a tweet storm are important.

That also raises the question of whether a paragraph should be restricted to 140 characters (assuming that's the see more limit) so it doesn't create a see more tweet itself when retweeted, not sure whether that matters.