I think that is the point precisely!
Terrorists et al. dont want to advertise if they are using a private flight. If Special branch know about yr flight and whos on it..they will be more confident that you or yr pax are not terrorists.
Think of it as covering all options. Commercial airlines have these checks too, and if you ever walk off a Belfast flight and see men/women in suits watching each pax..thats them! Its just they are very inconspicous...trying to avoid being noticed!
Im all for making private flying more easier etc. but these guys arent solely out to ruin yr day.
BTW even if you dont need a passport, id advise to take it if you have one!