A risk versus benefit proposition?
Statistically unlikely to have a high altitude rapid decompression event.
High likelihood that wearing the mask when supplemental oxy is not a physiological requirement introduces other safety threats. The discomfort and distraction of wearing the mask probably more than offsets any potential safety gains. Particularly with the integral smoke goggles!
One thing though. Given the TUC at FL410 and above, (never mind the shock of an explosive decompression) pilots should be able to consistently demonstrate that they can don/seal the mask and select emer 100% in half the TUC. Incorporate this into every training/checking event. If one doesn't practice this with regularity, they may not be able to do it when needed. I've seen many co-workers royally fail to get it done when they're not expecting it. (during training, thankfully not the real thing!) The first time I saw this, it was me that needed the practice.