Jeff,
That's a really good question but I think the truth is almost the opposite of what you suggest.
There was a time when most of the ratings seemed to be driven to a large extent by the fact that a course hosted top tournaments, or other pro events, and courses like Medinah got a bounce from that cache.
I think ratings have gotten much more sophisticated in the past 20 years, with more of a focus (at least in Golfweek and Golf Magazine) seemingly on what courses are best for EVERY level of player.
I think most raters know that if Cypress Point or Pine Valley hosted a major medal tournament the overall scores would likely be pretty low. But, I think that most raters feel that the enjoyment those courses provide to the other 99.999901% of golfer overrides those seeming deficiencies.
Put another way, if Medinah was still as enjoyable to the largest percentage of golfers, as say...NGLA, Fishers, Sand Hills, etc., and got toasted against par in a major, I don't think you'd see it's standing diminished in the least...not in the rankings, nor anywhere else.
Witness, exhibit A, The Old Course.
I rest my case.