New drones or not, lose one piranha and you can replace it when you R+R, which is the real benefit as the unit comes back full strength.
And don't forget seeker missiles. They'll allow the piranhas to skirt the table edges with 60" homing ignores cover crack missiles, that you can just replace. And now that they can still fire at bs 5 from a markerlight even if they boost or jink, what's not to love?
I think you're definitely right about the drones being able to rearm and refuel, the tyranid FAQ tervigon ruling backs this up IMO. I'm not sure you could enter by deep strike though? I'm leaning the other way on that myself.
Well that's a Tyranid FAQ. As long as there is no BRB FAQ or Tau FAQ that states otherwise it's just an assumption. RAW it's still possible to deep strike for everyone except for the units in that Tyranid formation.