True music, however, is about contrast. Beautiful pieces of music contrast multiple musical elements, including rhythm. Once the human ear has heard too much of any one thing it becomes bored and begins to tune out the content. Constant contrast hold the listener's attention and challenges him or her to discover what will happen next. These facts hold true for improvisation also, since it is a form of musical composition (the variation being that it is spontaneous).
Attention to rhythm in improvisation can fulfill more than just the role of achieving rhythmic contrast. In addition, motifs can be expressed through rhythm as easily as they are expressed through pitch. A particular rhythmic motif can be expressed across phrases of static pitch, stepwise motion or leaps in pitch. In these instances the rhythmic motif can be a commonality guiding the listener's ear through phrases of varying pitch content.
In my own practice, I use two methods to attempt to enhance my expression of rhythm through improvisation. For practice in rhythmic variation, I rehearse using only one pitch per chord while improvising. With this method, I force myself to use only rhythm for my source of variation. This method is also useful when rehearsing dynamic and other elements of variation. I gradually expand by introducing two pitches to my pallet, and then three. The whole time I maintain focus on rhythm as the primary element of contrast.
When practicing the development of rhythmic motifs, I write out one or two rhythmic motifs and try to implement the in improvisation across a wide range of pitch content. I usually start with a narrow pitch range (even static motion) and then expand to a broader range of motion. I save these rhythmic motifs as content that can be used in future performances. I also listen for interesting rhythmic motifs in other people's performances and try to write them down. This is easier to do with recordings, but I have walked away from concerts with napkin notation in my pocket (whatever works). The main idea is to constantly be expanding our ability to express music through every element instead of just focusing on just one like pitch.