Did it once a while back after reading some of the first reports about umami for the same reason's the OP is thinking.
Didn't really notice much of a difference in the beer except a sour salty thing. Never saw the need to repeat the trick.
That sounds about right... I don't even like the effect MSG has in most prepared foods (and I can always tell when it's in there). I think both the frenzy over the healthfulness of it and supposed allergic sensitivity to it (which is in reality very rare despite people's paranoia) is overblown... but by and large I still think it is a totally dispensable ingredient for any food, and the contribution to beer flavor would likely be akin to a salted meat broth. It may be naturally derived, but I don't consider it a natural ingredient and probably wouldn't want it in my beer anyway.
Then again, on the other hand maybe I should follow the advice I'm always dishing out and just try it before I make a judgement... I guess one could add some to a single serving of a finished beer to assess the affect.
Still, it seems to me that MSG in beer would be just as bad as putting saccharine or other artificial sweeteners into it.