sry but i doubt this would work well,
since you can increase bounty whenever you feel like it you can let the pirates stay somewhere until you are happy, be it costly,
the further in the game you get, the more powerfull pirates get, but if you have little bounty then suddenly you don't need to worry much because they'll leave anyway, (just send a couple a scouts and done)
correct me if im wrong, but i always felt that you should always worry for pirates, if they come for you you should be ready for them and if you are not, then why the hell didn't you? (im not talking about the actual situation of it as it is now, but as i think it should be)
planet defenses arent great, unless you max it out and the pirates are still fairly weak pirates should be able to take those defenses out if there isn't a fleet around to aid the defenses, (well, save for a nicely upgraded vasari starbase perhaps)
if you let bounty control the pirates after they have set out you basicly allow the players to decide what the pirates do, which shouldn't be the case,