While this certainly brings more security to the performance (and without questioning the advantages of such playing), I feel that this is sometimes done out of fear of risk. It feels risky to let go and to make music in a little more relaxed way. In such performance, anything can happen - ritardando can be not quite what you want it to be, articulation might not be always very precise.
Such rigorously choreographed performances sometimes are also done out of fear of failure. We are afraid to make mistakes in performance, so we memorize everything to the utmost detail. When we are afraid, we feel vulnerable. When we feel vulnerable, we think that something bad might happen to us.
Consider this for a moment - if many written organ compositions of the past were considered as models for improvisation for future generations, perhaps allowing more freedom into the performance would be also appropriate?
Yes, having some spontaneity in a performance might lead to feeling insecure but it also might lead to certain moments when the music comes alive and touches other people in the unexpected way.
Have you experienced such moments in your past performances?
PS One of the reasons some people don't improvise on the organ is because of the same feeling of risk and insecurity. And that's exactly why other people choose to improvise nonetheless.
[Thanks to Marcel for this subject]