Adult amateurs provide a privileged window into the experience of young learners, since many of them quit studying early in life -- usually during adolescence -- and resume later out of a feeling of having missed some important element of life. At whatever level of accomplishment--sufficient to play Beethoven, Brahms, or even Rachmaninoff--the aspect of music that most eludes them is rhythm. Trained to think of themselves as inaccurate they self-impose a metronomic standard.
This has two unproductive consequences. For the solo player it obscures response to particular sounds, usually intervals or triads. In chamber music it makes communication with another player extremely difficult so that playing frequently becomes a competition as to who is right.
Hanging on to a rigid beat as if one's life depended on it is defensive. The opposite, which had better not be offensive, is open-minded. Mime master Etienne Marceau used to demonstrate the difference by showing the back of his hand as opposed to extending the hand palm first.