I understand how you feel Hub - I used to think my wife's allergies were more "in her head"....until we wound up in the emergency room 
. I have seen the evidence for both sides of the MSG argument - and, unfortunately experienced one side of it.
It was not a immediate thing - but years of exposure that created a "sensitization" to MSG for me. Not exactly like an allergic reaction. Kind of hard to believe...until it happens to us. 
When I was writing the article series on Appearance, Taste and Tenderness that KCBS uses for some training, I was very surprised to discover in my research that MSG is a natural substance that normally occurs in some foods. I always supposed it was an "unnatural" chemical additive. It is a super-saturation of it, when it is used as either a tenderizer or a flavor enhancer or both, that is the real culprit. I've read that Frito-Lay has cut way back on using it (was a major contributor in their potato chips) and it isn't uncommon to see "No MSG" signs in some restaurants, particularly Chinese.
Aside from the well known sensitivity and reaction problems, I've found that rubs that are high in MSG will cause some texture/tenderness problems (instead of solving them) in some barbeque approaches. The MSG emulsifies the outer surface of the meat. Then, depending on how the meat is cooked, it will either stay mushy or develop a "super bark" that isn't exactly pleasant to the mouth feel. These effects are most pronounced when the rub is left on for an extended period prior to the cook.
Back to the "Tenda Rub" -- I'm really curious to know what's in it. If it is MSG I'd most certainly stay away from it. The only other natural tenderizer I'm aware of is acetic acid (which creates the powerful sourness of vinegar) and I don't think that would taste very good. I really am curious. Maybe somebody will "guinnea pig" a package.
Hub
