My understanding of the 737 NG WAI system is....

a/ Works well on the ground as well as inflight.
b/ The outboard LE Slat (on each side) is not heated at all.
c/ The anti-ice system works with slats in any position.
d/ On the grnd, the system is temperature sensitive, i.e., when they get up to the thermostat temp setting (either side), both supply valves close, and cycle around that temp. In flight, the thermostats play no part.
e/ When takeoff power is set, (beyond config switch setting) both supply valves close (and both blue valve position lights go bright), the switch stays ON until liftoff, when it goes OFF, and the blue indicator lights go off. Further normal inflight operation is at the discretion of the pilot.
f/ If an RTO occurs, when the thrust levers are returned below the takeoff setting, both valves will reopen and continue to be controlled by the thermostats.

Works well, costs a lot of money!!

EW73