1. The same people who can't judge a brake pad by looking at them probably aren't going to know which points to measure between. [Though the common error here would be to assume there is more brake pad left than is the case, by incorrectly including the backing plate in the measurement.] If the min spec is 4mm and you measure 4.5mm, they're clearly legal/safe, but for how much longer? If you don't know how a new set of pads measures, you have a hard time estimating.

2. This is not in the context of "no one knows whether these pads are above minimum spec" but rather in the context of a dealership recommending service before the pads have worn down to minimum spec. There are valid reasons (mostly convenience) why someone might choose to have brake pads replaced before they're at minimums and valid reasons why others would choose to run them all the way out.