There is probably no more violated FAR in US operations than this one. This is unfortunate, if for no other reason than it sets a standard of non-compliance which has a demonstrated tendency to migrate.
In my own experience, I recall having to assist the first officer in setting up his comm panel during an actual cockpit/cabin smoke event. Now, before everybody goes off on a tangent about training, let me say that first, I agree, and second, if you donned the mask and set it up everytime you were required to, it wouldn't be a problem. The day you need it is not the day you want to find out what you don't know about it. (By the way, he wasn't a complete neophyte...after we got his comm problem solved, he proceeded to run the checklist, troubleshoot the problem and stop the smoke, based on an idea that I am quite sure I would never have had.)
Along those lines, it may be wise in the case of someone leaving the cockpit to at least pull it out and see if it is all intact, disentangled and so forth. I frequently find Eros masks on my current mount to be, well, showing obvious signs of not having been used recently.
I also had the fun of a simulator exercise in the 727 some years ago. We ended up landing it in manual reversion...with the masks and goggles on. What struck me then was that, as in all such situations, the more familiar one is with the equipment and with the difficulties involved in using it, the lower the stress level encountered. It has always seemed to me that the only way to get comfortable with using the mask is to use it regularly.
That said, the point regarding total oxygen quantity on long haul is worth considering. It can certainly be argued that some authorities have not kept the regulatory requirements in step with contemporary design and incident data.