[benbristow]

Well it's still using JS to send the AJAX request, so in-fact something along the lines of 'on text change, set some element text to text.length' is a lot less JavaScript code than 'on text change, format and POST an AJAX request and then set the element text to a value within the response'.

[RussianCow]

The difference being that the code to send an Ajax request an update the DOM based on the response is already coded and already used in other places, whereas updating the text length client-side would have to be custom code. Not that I'm arguing one way or another, but the reason it's done that way is to reuse the same set of primitives for everything.