The torch on the front of the Redstone Comparator produces no light when toggled on.
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Confirmed in 13w02b.

Hmm... Isn't this interesting. Compare with MC-5735.
I'm not sure the front torch is meant to give off light, but it seems the rear torches certainly are meant to (no clue why that bug was locked; it wasn't a duplicate): give yourself 149 and 150, and place both. Both are comparators, but 150 emits light and has its rear torches on. If 150 loses power, it becomes an ordinary comparator. It seems the game is not using 150 for the lit state, despite its existence. Note that the front torch has no bearing on lighting for either state, and no light-producing comparator with only the front torch on exists.
In fact, 150 has a unique graphic as opposed to 149, which merely has the ordinary comparator graphic with lit torches. So, even if the lighting is intentional to reduce lighting updates, it does not seem intentional to leave one of the comparator textures unused, after updating its appearance.
Confirmed.