Am not sure I understand your thoughts about the wing inspection light. I think most of us, when flying an aircraft where it is possible to see the leading edge in detail from the cockpit, would very much like to do so. I do not see how you can describe keeping an eye on the wing in icing conditions to be a 'bad habit'?!
To think that because you have your ice protection systems operating you have nothing to worry about seems a rather dangerous attitude to me. Whilst the modern systems are very capable and well designed they are not infallible, and there are conditions out there which they will not cope with. The leading edge can be a very good early indicator of this and when you need to get the hell out of there.
I think he means simply that if too much focus is put on monitoring a system that (in his experience) functions properly on its own rather than keeping more focus on a changing situation up front, it could be a very plausible potential reason for overlooking a speed decrease. Not unreasonable to call that a bad habit.
Obviously sans CVR this is all guesswork, but when a Q400 captain flying in the Alps chimes in to say that in his experience the aircraft itself handles icing well but crews are prone to displaying certain behaviour which might distract them, it seems worth considering.
Combined with Otto's anecdote two posts later which shows very vividly what can happen even in good weather and simpler conditions, this direction of thinking seems to be more probable than some of the more obscure mechanical or technical failures put forward in this thread.