Actually, councillors are being forced to vote a certain way. It is not acceptable to turn down any candidate for whom a vacancy exists, as long as they are not disqualified. So by allowing councillors to be selective, they would be failing to fulfil the requirement to have a full council as soon as practicable. There is no provision to exclude " less desirable" candidates. Recent legal advice was that a vote must be held, as that is how councils resolve an action. However, there is only one acceptable way to vote, and that is to co-opt, to do otherwise would be to risk judicial review.
Personally I am extremely uncomfortable with the notion that any one can instruct anyone else how to vote. Much safer to consider ammending standing orders to reflect that when there are more vacancies than candidates, co option does not require a vote.
I recently was co opted to one of 3 vacancies after a humiliating 25 minute discussion that risked every participant's adhesion to the Nolan principals.
( I had been a vocal critic of a previous councillors activities that led to the council acting unlawfully, libelous communications were made in an attempt to keep me out. I was not disqualified, So a person with skills and good intentions could be kept out by people wanting to maintain a status quo).