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I believe the real reason behind the PVA bonded safety glass might be to minimize reflections between interfaces.
Consider light emanating from a source outside of the set and falling on the screen:
In a set with a 15GP22, you have the interface between the air and the safety glass, an interface between the safety glass and the air behind the safety glass, and interface between the air behind the safety glass and the front of the tube, an interface between the front of the tube and the vacuum of the tube, and finally between the vacuum of the tube and the phosphor dot plate. There are going to be reflections at any interface where the light passes from a low index to a high index material. This is basic optics. In the case of light that makes it to the dot plate, there are now a bunch of low-to-high interfaces on the path back out of the set.
All this reflecting has the effect of washing out the picture and reducing the apparent contrast.
The later 15HP22, 19VP22, 21AXP22, 21CYP22, 21FBP22, etc, eliminated the interface between the vacuum of the tube and the dot plate by depositing the phosphor directly on the glass that makes up the front of the tube.
Now consider that the refractive index of PVA (1.48) is much closer to that of the various glasses (1.40 to 1.60 for most varieties) than it is to air. You would be replacing a air-to-glass interfaces with PVA-to-glass, and since the indices of refraction are much, much closer, you would minimize the reflections to some degree. Granted, there would still be reflections at the interface between the PVA-to-glass interfaces. Even glass-to-glass interfaces (with two seperate pieces in direct contact) must exhibit some reflecting as the structure of the glass itself is not continuous and there are gaps and voids between them, etc. Still, the severity of the resulting reflections is substantially lower.
The PVA bonded tubes should give less "glare" from ambient light than a 21FBP22 set. I can say that there is much less glare on my CTC-7 than the Westinghouse H840CK15, and I suspect that a CTC-16 with its bonded tube would exhibit less "glare" than say a CTC-7.
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