The reason that radios need to be certified for Part 90 for public service bands is to be sure the radio does not put out spurious emissions. Ham radio gear is designed by the manufacturer to only be used in the ham bands. The circuits and final output is designed to only be used within a certain spread of frequencies. When you start using frequencies that the radio isn't designed for there are unknown issues. Out of band, spurious emissions can come from the radio on frequencies other than the one you're transmitting on. Your radio display may say 155.000 MHz but you could be putting out several watts of power on 158.000 MHz, or 138.000 or 154.500, or anywhere else, which could cause intereference to other users and services. The Part 90 certification ensures that the radio is up to specs within the designed frequencies and ensures that the radio isn't causing harmful interference.