Optician locum insurance is not cheap. When you desperately need a locum optometrist for your practice in the UK you are looking at around £300 a day, so that’s £2,500 for a locum for a fortnight.
The problem with requiring locums is that, apart from pre-booked times such as holidays, you really have no idea when you would otherwise need one or for that matter for how long you would need one. If somebody in your practice goes sick, it might be for a short while, or it could be for weeks.