So hands up who has a 'fully optimised signal chain'?
I would have thought you should use the same brand of cables, pedals, strap and plectrum for sonic sartori.
Certainly possible with Fender.
Seriously though, I would expect most same-manufacturer amp/speaker combinations to sound decent, but obviously there are exceptions - a rock oriented amp may not sound at its best when used with the same brand's acoustic-optimised cab.
But ultimately, amps and cabs do very different jobs so mix and match has the potential to be better, allowing for the fact that better depends on the musical style, where and how the setup is being used and the preferences of player, group and audience.
It's like saying one brand of camera works best with the same brand of lenses, when it's clear the best lens for a particular situation will often be by a different manufacturer.