I believe the JAR's (LPC) require a preflight instrument crosscheck. In Ryanair we check all piotot/static driven instruments plus Heading (via ADIRU's) before pushback. Also RVSM (maybe not in the ATR)requires certain altimetry tolerances which may be detected with a good instrument crosscheck. There is a good comparator system on all new Boeings that will produce alerts to the crew if instruments are in disagreement for longer than 5 seconds.