The problem with IDLE is that it requires one TCP connection per folder you want to watch. The better alternative is NOTIFY, but in my experience, it is not as widely supported by server and client implementations.
But that is an extension so you cannot count on it being there. Thus a well-written client would need to handle servers that do not support IDLE. The X11 protocol is similar in that it has a lot of extensions that are theoretically optional but de facto required to deliver a good user experience. It makes implementing such protocols complex because you need to duplicate code paths, one for when the extension is present and one for when it isn't.
IDLE isn't sufficient which is why Lemonade was defined
https://tools.ietf.org/html/rfc2177