If we improvise with an instrument, tool, or idea that we know well, we have the solid technique for expressing ourselves. But the technique can get too solid -- we can become so used to knowing how it should be done that we become distanced from the freshness of today's situation. This is the danger that inheres in the very competence that we acquire in practice. Competence that loses a sense of its roots in the playful spirit becomes ensconced in rigid forms of professionalism.
The worst possible outcome, in my humble opinion.