They are pointless. But not really much different to other parliamentary systems where the head of state is just a figurehead (eg Ireland and Israel, as opposed to the USA and France). An elected figurehead would be far preferable, of course.

It would be a very fucking terrible idea to have a monarchy with actual power which actually used it. Almost all of the powers technically held by the monarchy are wielded by the govt on its behalf (eg the Royal Prerogative) and that is bad enough. Putting those powers directly in the hands of the hereditary monarch (a multi-billionaire with delusions of grandeur) would be much, much worse.

There is a 'royal stick', as you refer to it, which is the only power they have to wield unilaterally but it is almost impossible for them to actually do so, and definitely not as often as "once a decade or so". If they think the govt is becoming a dictatorship such that there is no other way to stop them, they can step in and force that govt out. But in practice they would need the backing of the entire British establishment to do so and, in reality, any British dictatorship would likely have the backing of the monarchy anyway.