As there are plenty of peer-reviewed papers showing the usefulness of the WG, there are also plenty of papers showing that the WG is deeply flawed. I know someone who graduated with a first in PPE and received the academic prize for the highest mark. He always used to score 20-25 out of 40. I don't think this person lacks critical thinking skills, as his degree was PPE. He also got a TC at a well-known firm.
I was also told by grad rec from the firm where I did the vac scheme that they use the WG to filter out the number of app (for example,
Linklaters rejecting people with 38/40 and taking people with 25/40).
However, I agree with your point about firms using to measure your critical thinking skills. But that doesn't necessary mean that the WG is fully reliable. There is plenty of evidence which points to its flaws. Also the sample size is too small to infer that it is indeed a reliable tool of recruitment. I simply think that it's used to cut down apps. I have been told this by grad rec, associates, etc. But again, it comes down to your own views on it,