the resistance of an RTD probe should be extremely solid and linear. if you calibrate it once, it should remain the same almost indefinately. there is no real need to check against another thermometer.
and then you get into the problem of- if you have an RTD and an analog temp probe that do not match, how do you tell which is correct? the only almost-guaranteed-accurate analog thermometer are the glass alcohol-type ones. the analog dial gauge thermometers rely on the either gas pressure or metal memory, neither of which are extremely accurate, and can be influenced by outside sources.
I trust my RTDs over just about any other thermometer in the house.
thermocouples are a different story. some cheaper ones do have considerable drift, and since thermocouples rely on a measured voltage instead of a measured resistance, that voltage can easily be partly lost in transit to the controller.