Time over time again, I have seen crews putting faith on the " wiper/bolt" ice indication to estimate wing icing. This is completely wrong and extremely dangerous. Still, some people even teach this during line-training
It seems the instructors are putting too much faith into the FCOM info that states:
Wing Anti-ice Operation - In Flight
Ice accumulation on the flight deck window frames, windshield center post, or on the windshield wiper arm may be used as an indication of structural icing conditions and the need to turn on wing anti-ice.
It seems pilots seem to use to wing light more to put it in your face than using it for its original purpose.
That said,newer models do have an icing caution light to alert crew while icing is being accumulated.
I believe the 737 was certified up to 3 inches of ice without controllability issues.