If there is high turn over then the length of the learning curve is more important than the shape of it. Let's say there are two apps each with an employee starting, one requires two full days of training to be 90% proficient, the other allows for an instant start with a more gradual learning curve.

After 30 days of employment the first has spent 28 days at 90% efficiency, the other has had 7 days at 60% efficiency, 7, days at 80% efficiency etc.

A gradual learning curve could be much worse for companies with a high turn over.