The 3 deg Temp-Dewpoint spread is part of the definition of "visible moisture" in our FHB (747 Classic/GE). We use nacelle anti-ice in that situation.
Since the air pressure drops somewhat as the air is sucked into the intakes, it is possible for moisture to condense and freeze as it cools below ambient. At that temp there is seldom a problem with any reduced thrust due to A/I, so it's a good idea to use it.
Some engines/installations are more susceptible than others. Follow the guidance in your books.