In theory a release candidate would prompt all serious DokuWiki users to install it, try it with their templates, plugins, configurations and pages and report back all the little things we had missed. Then those bugs would be fixed and a proper, basically bug free release would be made.
That's the theory. What happens in reality is that everyone just thinks "oh it's only an RC - I'll just wait for the real release". Of course this has the effect that the bugs will only surface when people update for real, requiring a hotfix release soon after.
So as sort of a workaround for that problem we decided to not have RCs at all anymore but instead just deal with creating hotfixes after a few days. This of course may lead to people not installing fresh releases anymore because they expect a hotfix soon... there's no escape.
People simply don't want to be the ones who discover bugs. But developers will not catch all the bugs. Some of this can be mitigated by automated testing and we try our best to do that, but in the end there's just too much going on in real world setups to test it all in advance.
How could this be improved? We always announce when a release is immanent. And there are nightly snapshots available at our download page. So, if you want to help, just download that snapshot and try it. Report things not working. Especially if you're a plugin or template author: follow the core development and update your stuff before a release!