In the context of glazing oils, the discussion often is really about aesthetics concerns, and not really about protection.
Museums and art galleries are not always the best places to see how framing meets recommended standards and procedures. Curators and museum conservators are just as prone to not keeping up to current technology as are framers.
Even though CCI recommends glazing and recommends a solid protective medium for the verso, it is not uncommon to see evidence of traditional thinking still being employed in reputable museums and galleries.
It is also not uncommon to witness these same museums and galleries elicit traditional concern about allowing oils "to breathe" while at the same time, the oil-art over which their concern has been expressed has had its recto jammed up against the rabbet bead -("We've always done it that way.")
Depending upon the thickness of the impasto, oils can take many years to cure. While they are curing, their surfaces are "softer" and are prone to mechanical damage and to foreign matter adhering e.g. dust, airborne solids. Glazing protects the surface.
In the past, oils were not glazed primarily because it was thought that they needed unhindered ventilation to aid in their curing process. The same school of thought promoted the open (holed) back or perforated paper finish used to close up framed oils - BUT, in the past, airborne pollution and contamination was not a major consideration.
It has been demonstrated that chemical curing will proceed whether or not oil-art is freely ventilated. Solvents that evaporate during the curing process can and do escape through the fibrous mass of matboard filler, dustpaper, Tyvek, etc., unless an impervious barrier has been installed e.g. MarvelSeal and ordinary foils.
It is possible that the rate at which the solvents evaporate may be slowed down by installing glass, but a slower cure time usually mitigates the tendency for impasto and heavy layers to crack - rapid drying and curing will often encourage cracking - that is why artists are taught not to force-dry oils.
Prudence suggests that if impasto is exceptionally heavy, the art should cure for an appropriate period of time in a controlled environment before the art is presented for framing. This is the artist's responsibility.
It is the c-p framer's responsibility to ensure that nothing done to the art while it was being framed will alter its condition.