We would completely get people turning down things sometimes but a pattern of it isn't good.
We expect for example that people will apply to multiple entry level jobs (eg newly qualified), accept a couple of interviews because they will be trying to make sure they a secure a job. We get a bit sad when they reject us (because we are excited about having them!) but it's common place and not a big deal.
The same when people moved area. I applied to 2 jobs, went to 2 interviews and got both. Its completely reasonable that it happens once or twice. I wasnt going to turn down interviews without a solid offer
However if I've interviewed you before (or been involved in shortlisting where you might not know I'm even involved) and the reason given wasn't good, I would be dubious about wasting another interview slot.
I would expect questions about work patterns, terms and conditions to come fairly early on. I realise that's maybe a nhs thing as some private roles you'd maybe want salary negotiations etc
It would really depend on the reason, but you couldnt keep applying for similar roles. Pre interview i wouldn't blink too much at it unless I repeatedly saw the same person, but by interview you've invested a bit of time in it.
The way nhs interviews work, you're not always interviewing a mass of people so you might have pulled people in from different teams/bases etc and I'd be annoyed to do that for a morning to find it was simply to get interview experience.
I wouldn't be like furious, but I'd just think "we've interviewed that person before, they always turn it down, or things we can't give, so I won't bother interviewing this time"