Training to fly two very similar planes can be more dangerous than flying to completely different ones.
For example, some marks of Spitfire had engines that spun in the opposite direction to the rest of them. The aircraft were otherwise similar.
If a pilot gunned the engine during the take-off roll, the torque reaction will cause it to swerve. The pilot can catch it with the rudder if they are expecting it. If the engine is spinning the wrong way, and the pilot inputs opposite rudder, the aircraft will wreck it's undercarriage and roll over on the runway.
Whilst training to fly any variant, when a pilot has experience in other variants is obviously going to be quick and easy, expecting them to swap back and forth between variants is also clearly unsafe.
For example, some marks of Spitfire had engines that spun in the opposite direction to the rest of them. The aircraft were otherwise similar.
If a pilot gunned the engine during the take-off roll, the torque reaction will cause it to swerve. The pilot can catch it with the rudder if they are expecting it. If the engine is spinning the wrong way, and the pilot inputs opposite rudder, the aircraft will wreck it's undercarriage and roll over on the runway.
Whilst training to fly any variant, when a pilot has experience in other variants is obviously going to be quick and easy, expecting them to swap back and forth between variants is also clearly unsafe.