I used to loathe these with a passion (never went down the route of any graduate assessment centres as they might have used them, and now, quite a lot later, I regret that). Then I had a job where I had to act as guinea pig test subject for colleagues who I already knew, and who were training, or re-registering annually, in the proper assessment, administration, and feedback procedures.
There are many more options out there than Myers Briggs.
For types where your responses map you as where your strengths work as part of a team, it is a reasonably good tool for a team (it's really not all about you, and others should see no personal questions or achilles heels on your eventual report). Often, you see that a manager may have a habit of gathering people with very similar strengths around them, and have some big gaps where they don't deeply understand different natural styles of working, communication, etc.- therefore, that could be a weak spot in an overall team. To resist these strenuously is your prerogative, but I agree that it will probably be seen as quite anti-teamwork and anti-cooperation with the manager, which isn't a great leg to stand on.
Do they dumb down? Yes. Is it just a consequence of how you felt on the day? Yes. Better to talk to colleagues and draw out their hidden depths? Yes, but does that happen to the extent that it opens the gates of a golden future nowadays? Are they frightening? Perhaps at first but not really. Will they help you? Maybe but mostly just 'meh' - neither life changing nor destructive. There won't usually be any extremes or shocking revelations, honestly.