To be honest, for the majority of customers there is no need to change any of the QoS settings as there is plenty of bandwidth within their switches networks and the default settings on the MX are fine.
Very simply put: if there‘s any need to implement QoS, you‘re gonna have to implement it network-wide. So yes, it isn‘t sufficient at all only configuring it on your switches.
On the other hand, I‘m completely with @PhilipDAth here: in most cases, it hurts more to implement QoS as „best practice“ than just leaving it alone. Especially if you don‘t have a special need to implement it.