NB - When you have flap extended, its possible to get icing on the leading edges of the flap aerofoils as well. Seen this after landing in icing conditions many times on both Boeing and Airbus aircraft.
Bottom line - If your slats/flaps ice up, your high lift devices may jam making landing or even diverting limiting. Hence the recommendation to not hold in a config other than clean (in icing conditions)
ps. What jets don't have leading edge deice with the slats extended? Are there any??