I have read all the above posts, just about all are valid. But -----

After playing with live muscians for many years I did a job with an electric drummer, my guitar and voice. I learned quickly that the drummer did not follow me, I HAD to follow it.

My emotions and feel had to be put on hold. If I became excited for a moment I had t back off. If I am playing an up-tempo, let's say at 175, toward the end of the song I should be a bit higher, it's a natural feel. I do not feel this is true for latins or some contempory music. When the excitement comes the tempo must vary to compliment the emotions -- it can go back to the original tempo, but it should change to reflect.
The change is very small and should be felt. The dancers should not be aware of a change in tempo.

This can not happen with the arranger keyboard unless you program it in a sequence. But then your emotions must be turned on as the arrangement dictates. your emotion are now not yours anylonger.

My little bit, John C.