As Peter mentioned above, the effect can be both visually and aurally distracting to others in the band. I remember playing on a brass band course many years ago where the principal cornet player had a pronounced foot tap - sitting on a stage raised up from the audience, the front end of his enormous boot flapped up and down the whole time, right in the audience's eyeline, and the noise it made on impacting the floor was clearly audible. That's an extreme example, but any motion readily observed by others suffers from the same potential problems. And as Peter also mentioned, if that seems distracting, consider how much more so when the tapping is out of time - perhaps a consistent fraction of a second behind the beat (I've seen that...), perhaps pretty close to random (I've seen that too...). I take the view that, in our brass band idiom at least, the best way to feel time is to rigidly subdivide in the head, to develop a real intuitive sense of the structure and flow of musical time without depending on the crutch of a beating body part. But, if one really feels that one must, tapping one's toes inside one's shoes (as again, Peter recommended) risks nothing musically. Note though that this is not a truth for all musical idioms. There are highly-respected jazz teachers that will advocate the use of foot-tapping in their chosen idiom as a method to develop one's sense of time. It has its place. But that place has to be carefully chosen, and tends not to be within brass banding.