The noise of the slats operating as the aircraft approached the stall was most noticeable and probably saved the occasional life. I understand the reason for some operators removing the slats from operation - either having them locked or removing them from the aircraft, was maintenance costs. In my opinion, it was false economy. There have been several fatal accidents in Tiger Moths in Australia where a stall occurred at low altitude in the circuit. I believe that in some instances the slats of some of these aircraft had been removed thus negating the advantage of early stall warning. That said I can offer no proof of this statement
Providing the pilot accepted the Pilots Notes recommendation to lock the slats while taxiing thus preventing their banging against the leading edge of the upper wings when taxiing on uneven surfaces as on grass fields, then I would have thought maintenance costs should be minimal. Of course the slats should be unlocked for takeoff and landing and locked before aerobatics. While stalling speeds slats locked or unlocked were only a knot or two, it was the stall warning noise the slats offered that convinced me of their value particularly for student pilots.