When there are insufficient candidates for election or no election is requested by the electorate to fill a casual vacancy, there's little option but to co-opt. Ideally, you attempt to attract several candidates but in reality, its rare to get people queuing up to come in! I'd not vote for a candidate I'd not met but then I get no choice in an actual election would I? My council's co-option policy requires the candidate to be there (if he/she is that keen, he/she will be there!) and to give a presentation on what they can bring to the council in terms of experiences, life skills, background etc. and then there's a vote as there is a requirement for the successful candidate to achieve a simple majority. They're not co-opted if no one votes for them.