It ends up being mandatory if you're sharing a non-MIMO link with other systems that are using large packets.
I understand. I have architected networks for over a decade now. The real issue is serialization delay. If I have a tiny voice packet that has to wait to be physically transmitted behind a huge dump truck packet (big), it can still be a problem even with high speed links with regards to microbursts.
A single gigabit link can sends something in the order of 80,000 packets per second. If packets had a 9000 byte MTU, that would still be 12,000 packets per second. Having your smaller packets wait a at most an extra 0.02 milliseconds to be serialised onto a 1 gigabit physical link seems... rather unlikely to be a problem in the real world?