My understanding is that below 40 degrees most clouds are made up of ice crystals and therefore pose no engine icing hazard with climb or cruise power set. I'm not sure why this changes in descent. It may have something to do with idle thrust, slower airflow through the engine, etc. Any thoughts?
On the CFM and the JT8Ds only the cowl is heated. We have no way of protecting or anti icing the fan blades (other than increasing thrust) but I would be amazed if you could get fan blade icing at the power settings used during flight. The fan speed and centrafugal forces are just to great to allow ice to form. The only time I have ever encountered it is during prolonged taxi or ground holding in fog at low ambient temperatures.
I do remember some hoary old captain remarking that the definition of icing conditions wasn't always +10/-40 and so it is possible that these figures are quite conservative.
On the 732 EAI is quite a fuel penalty. I doesn't appear to be so significant on the CFM so I suppose it's the usual. If there is doubt there is no doubt! Switch it on.
Just my thoughts!