Honestly, imo clinically in aggregate the actually score itself provides very little information beyond what a 5 minute conversation would achieve, and the result could be better thought as bordering on 5-6 level categorical variable rather than a gradient due to their biases and inherent individual patient variance on performance and test taking context.

The sub-sections of things like the WAIS can be of some value for identifying specific abnormalities or deficiencies, but as you said, is probably of more value clinically to split them out into separate tests/activities rather than to group them all together into an aggregate score. It's a bit like judging athletic ability and skill by BMI and fat percentage rather than just playing an opponent in tennis to find out if they're a good tennis player.