The airline that i work for operates a fleet of several leased 757's without winglets. The owners of the aircraft have blocked the proposal to install winglets for the sole reason of the modification will roughly half the resale value of a high hour 757 due to the fact that the alteration in wing loading transfered to the fuselage will not allow the large freight door to be installed in the event of a cargo conversion. Its very common for high hour 757's to be snapped up by cargo operators. The airline or operator would see some financial benefit but the owner will be out of pocket long term.