Hi Les,
From an enineering point of view rigid axle dive planes (non-retractible) are simpler, stronger and need less maintenance. Loss of surface speed fairly negligible. But they were a pain to watch in close quarters maneuvering. On the other hand they came in handy in occasionally slicing the hulls of some escorts in close combat - though the latter was hardly considered in the design and not a specially encouraged tactic. Cheers, RB