I understand where Alan is coming from with the MoT analogy but I take your point about legal requirements etc. In some ways its similar to an MoT in that calibration only verifies the performance/accuracy at the time of calibration. That's why the IET recomend interim checks to confirm on-going accuracy between calibrations. The CoP recomends that instruments are calibrated in accordance with the manufactures instructions and most (including Seaward) recomend annual calibration.
The amount of use does not have a significant impact on the calibration as changes in performance are normally due to component aging or drift rather than wear and tear. Our products do have some moving parts (relay contacts for example) but are designed such that these do not impact on the measurement accuracy.
The other factors is the requirements of organisations such as NICEIC, NAPIT etc who make recomend calibration or in some cases companies require calibration certificates as part of the ISO 9000 system.
If for no reason other than peace of mind, it's worth checking the performance. A lot of companies use checkboxes or other devices to perform on-going checks. This is generally good practise as it avoids finding out during an annual calibration that the accuracy has drifted and then pondering when did it go out of cal and how many historical tests may have been affected.