This is a fantastic thread! Rating/Slope should be required reading for every golfer. Unfortunately, I see many golfers that have no idea just what kind of impact rating/yardage/slope have to their golf game.
It seems to me that the courses in the area that I live/play in are regularly updated/changed. I belong to a country club and have been a member since late last September. Just in that time the rating/slope has been updated twice by the USGA/Maryland Golf Association. I made a comment not too long ago in another thread about how it seems that when someone breaks 80 that it is some kind of level or accomplishment that makes you a good golfer. The actual answer to that is that it depends.
A guy in my office recently shot a 73. Many people in the office were treating this as some kind of Ruthian achievement. The reality of this guy's 73 was that it was shot on a par 70 course that was 5800 yards, had just a 67 rating and had a 115 slope. So yes he was just 3 over par for the round, but he was actually about 7 over being "scratch". This resulted in an argument because many people believe that one that shoots par is a scratch golfer. Obviously this is not necessarily true, it is only true if par is equal to the course rating. The guy who shot the 73 was well aware of this too, so it was not like I was bringing him down. He was amused because all these people were treating him like he was much better than he was (side note - great guy who is my friend and is a good golfer, would be even better if he played more).
What is frustrating to me is that unless the course you are playing at was just recently graded, there is a chance that you are playing on a course that could be much easier or much harder than the rating/slope. To this I think that the rough of a course is a HUGE variable. As I mentioned, my country club was recently graded (in April). Since then we've had a lot of rain and the rough has grown. Thus it is much thicker/longer than it was when the course was graded. Also, since I play early Saturday/Sunday mornings most of the time, this same rough is also very wet/sticky in addition to being longer/thicker than when the course was graded. So while the USGA was airing commercials this past weekend during the US Open about how their handicap system is fair, I am calling bullsh*t to that. The handicap system is not fair at all. I started another thread about this (because my hc is higher than the rate that I am improving/scoring, nobody will play me and there is no way to adjust this, in addition I am being labeled a sandbagger by some at my club when it's not my fault at all). Per the USGA website a handicap is supposed to be based more on potential than average. Thus if someone shoots a score that is a hc rating of 9.0 for example then according to the handicap being based on potential, to make things fair, the hc should be equal AT LEAST to the lowest score or pretty close to it, yet in some situations the actual hc can be several strokes off the POTENTIAL.
Since all of this ties into ratings and slope, to answer the op's question, I'd say that in my opinion, most of the time things do not add up as they should. Golf is supposed to be a fair game, yet unless a course is rated very often for rating/slope, it is far from fair. There are also situations where a course is much easier than the rating/slope as the OP pointed out and this is not fair either. Someone can take advantage of that situation. From what I've seen though it seems that most of the time it's the opposite that happens: a course is rated in early spring before the rough gets thick/long and thus golfers who only want fairness are stuck playing a course that should be rated higher.
Sorry for my rant...this just really upsets me because I have only been playing golf for 1 year and 18 days. I was informed and learned that golf is supposed to be fair and since it's been a short time since I started playing, this is fresh in my mind. We are being told that it's fair and the USGA even runs commercials during the US Open claiming that it's fair when in reality it's anything but.