I think that's the distinction to make: 'General Purpose'. The Null Space Projector is a great thing to have when you need to crack a base ship or a starbase. It starts destroying components immediately and saves you 10 turns or more of breaking down shields. 10 turns against a starbase-sized collection of weapons is an eternity.
But it doesn't fire very fast and a bunch of little ships with plain old APB or MB will knock down YOUR shields while you're waiting for it to recharge. The PPB is a normal once-per-turn beam weapon that also penetrates first generation shields. Nice weapon, and I can see why some think that it is unbalancing. I can't think of any reasonable way to restrict it, though, except perhaps to make it harder to research. Make it Physics level three, maybe? Or require that Phased-shields be discovered first? That would make some sense.
[This message has been edited by Baron Munchausen (edited 07 February 2001).]