what do you instructors do when the student spots the same thing and draws it to your attention?
Personally I will always point out any defects to a student even if he does not see them, as Johe02 says it is good captaincy training. A defect will either be Go/no go or, "given my experience I am happy to accept this, but at your level more consideration needs to be given and is probably no go for solo flight" A good example is again the stall warner, any instructor who cannot recognise an approaching stall without a warner should not really be in the job (and many aircraft do not have a warner of course!), but for a PPL student with only 10-20 hours this may be a different matter.