What I've missed in the discussion so far is a bit in line with F/E Hoppy's remark on blowback. Wing Anti Ice systems normally only heat up the Leading Edge of the wing.
During operation on the ground the ice collects on the entire top of the wing during precipitaion and all the cold parts (top and bottom) of the wing during frost conditions. As we all can see every now and then during flight ice starts to collect at the leading edge, possibly spreading a bit further aft but hardly ever more than about 10 percent of the cord. Wing Anti-Ice systems are designed to clear this "In-Flight" buildup of ice. They do keep the forward 10 percent of the wing clear of ice while operating on the ground but this is not nearly enough. Before T/O I'd like to have the entire wing or at least the top free of ice, leaving room for a light frost on the bottom.
A proper de-icing treatment clears the entire wing and protects it for the length of the holdover time. If I then heat up the leading edge the glycol evaporates and I lose this protection of the forward 10 percent and trade it for thermal Anti Icing.
Apart form company rules that seem to differ on the subject: What do I loose, what do I gain? Anyone?
Sq