That is a bloody good question. Hopefully I can shed some light on it. If you go right back to wing planforms and all that shiit, you'll remember that an elliptical wingtip will provide the least amount of induced drag for a given wing. The same goes for a propellor. If the tip tapers, induced drag is reduced and hopefully the efficiency will balance or better the loss of thrust. A military aircraft such as the C-130 with its squared off tips is probably owned by a government who accepts loss of efficiency against dollars per hour. A C-130 produces lots of required thrust with an accepted loss of efficiency. A P-3 Orion however, has a different type of prop for two reasons. The P-3 was developed from a civil aircraft where efficiency was important, hence round tips. AND a P-3 requires efficiency to patrol the oceans for hours and hours on end.
My argument goes to pieces when you see a Convair 580 however.............................