Interestingly, do we have 'limits' that we impose over and above the authority minimum, or do we accept all comers that we consider suitable after the Pre Course ride?
Who are the "we" you are referring to?
If an organisation provides training for the Instructor Rating and the requirements for starting the course are x then provided that the candidate meets the minimum requirements you are not permitted to block their attempts to complete the course by adding higher personal requirements.
Letting them know that even if they pass the course they will not get a job with your organisation until they have certain experience is a separate issue.
If they guy meets the requirements to start the course, completes the course and passes the test then they are as entitled as anyone else to hold the rating.
Spending 100 hours flying circles within 10nm of the base airfield on CAVOK days may give someone 100 hours PIC in their logbook but it gives them zero experience.
At 78hrs P1 I was still merrily getting lost within 50nm of my home base, had 4 airfields in my log book and I didn't dare go anywhere near a stall
Is that a measure of your ability, the organisatioon that trained you or the examiner that said you passed the test?????
Who is going to teach to a higher standard - a natural teacher with a PPL or a 20,000 hour Badge collector who has lots of badges but nothing to back them up with and no teaching ability.
A good training organisation will be able to employ an FI(R) and provide appropriate further training and supervision so that they develop their abilities. It is at this stage that the foundations of a good instructor are built upon and when the instructing takes shape.
Unfortunately, far too many organisations think that the end of the FI course is the end of the training rather than simply the start of the most important stage.