Are exotic speaker cabinet materials overrated?

Exotic materials are overrated yes but they never hurt anyone just a cost that can be needless or necessary depending on who you ask.

I've seen inert standard speaker cabinets. and resonance filled specialised cabinets... looking at you Borresen

Since a speaker is basically drivers, a crossover, and a box they’re obviously all critical.  However, that said if something needs to be pared back I’d rather have higher quality drivers and crossover and get by with a well-built, lower-tech MDF cabinet as pumping big $$$ into an esoteric structure probably has much higher diminishing returns as to its overall impact on sound (huge $$$ to develop and build with relatively small improvements, and not that they still can’t be very meaningful).  Some of my fave speakers muddle through with very good but relatively lower-tech cabinetry.  Could they sound better with more advanced cabinets?  Yeah probably, but how much better and at what cost?  Always trade offs. 

The correct answer is of course, as always:  it depends. 

Speakers Drivers are transducers, which is a device to convert one form of energy into another, the Driver converts electrical energy into Kinetic Energy.

Energy is constant it is never gone, but transferred to take on a new form.

Energy Transferral is also to be passed into the Speaker Cabinet of Structure for the Speaker to function.

Structural Designs for Materials used, or Materials Selected or designed for the purpose of being the Speakers Structure, are done with an intent to manage energies transferred, to attempt to have a Structure that has the least negative impact on the Amplitude released into the listening space.

The intention being only one sound is received by the listener not secondary generated sounds.

Certain companies use a Resin Impregnated Densified Wood Board to achieve their ideal structure, and I totally believe this is a very good material selection.