There can quite often be ide build up on the leading edge with none apparent on the indicator. Also since the ice detector isn't de-iced, after you've used the WAI the two situations no longer correspond, even if they did in the first place. I prefer to look out of the window for WAI use.
EAI - follow the SOP, if you don't understand why, then you're not qualified to start coming up with your own bright ideas!