You can get rapid pitot blockage (or partial blockage) due to ice without any visible ice on the wings. I've had this a number of times, in hazy cold VMC. The plane isn't making the expected IAS for the power setting (and the fuel flow rate); switch on the pitot and hey there's another 5-10kt
Re icing, while the plane won't be cert for known ice, people still pop into clouds here and there, and a few mm won't (generally!!) get noticed especially once the speed is up. But if the pitot gets blocked, you have lost a very important instrument for landing.
I switch it on anytime the OAT is below +5C.