Actually in the right environment they can be extremely effective. In a situation where you have a coastline with many islands and lots of offshore traffic it is easy to ‘hide’ in the clutter.
Plus there is a certain quality in quantity. In coastal waters a handful of PB’s can be a force multiplier. Even though the platform is cheap, it still takes hi-tech weaponry to defeat the weapons they fire. And in the right situation they will not even have to have any sensor active, relying instead on data from other installations or ships.
With a SSM that outranges most shipboard gunnery systems they could threaten before being threatened and because of their small size many commanders would not want to ‘waste’ a high value SSM on them.