1. The level of experience gained from an apprenticeship should be a portion of the skill gained from the work itself rather than time based. In the "apprenticeships" I've been in it is not necessarily the hours one puts in but what is put in the hours, one can usually distinguish the difference of two apprentices with the same time experience from different instructors.
2. The best teachers are also good students, in an apprenticeship teacher and student skills would become enormously useful compared to their fairly limited application. Circling back around to the opening remark of section 2, being a teacher should give the instructor more experience per job completed of the amount I have limited opinion except that it shouldn't be huge but should be noticeable.
3. Increased speed of production, not only retrieving items for the job but the job itself, having someone to hand you the necessary tools can speed up the process surprisingly quickly.
4. Aptitude. For teacher and student both kids tend to learn quickly but some kids are naturally inclined towards certain skills and some teachers are better than others (see 1.) the role played should be larger than normally applied (in my opinion).
5. Finally, quality consistency. With the instructor voicing the steps in a process they will be less likely to screw up, mistakes are not consciously made so when the steps are being voiced it is so much harder to screw up. Also, without moving around and away from the workpiece the mind doesnt move to the small other tasks as much leading to improved skill overall (link to 2)
This is my 10 cents based upon personal experience, feedback appreciated.