Is there any known advantage to enabling VPLS CW if system MTUs are chosen to ensure that fragmentation is never required?

It would seem to simplify the PE routers packet processing path and save 8 bytes/packet turning it off.

Not 100% my area of expertise but I do seem to recall that if you have devices in the MPLS network that do hardware forwarding, there are some ASICs that try and introspect past the MPLS label and can cause some subtle mishandling of VPLS payloads. The phenomenon that I recall was this one [1] where MAC addresses with a 4 or 6 in the unlucky offset position can cause Ethernet frames inside VPLS to be incorrectly introspected as IPv4 or IPv6 packets and mishandled.

Since most of the packet handling in RouterOS happens in software, I doubt this is an issue on RouterOS and by extension wouldn't be a concern on an all-Mikrotik network.

[1] https://seclists.org/nanog/2016/Dec/29