Showmanship = physicality = just another means of expression = communication
Of course, it'll vary widely based on the quality of both performer and audience (such is symbiosis, when it exists), but the point is that "regular" audiences (i'e. - people who don't participate in music as a purely "academic" exercise of pure intellectuality and/or theoretical analysis) are more than capable of receiveing messages from visual as well as from aural stimuli.
A performing artist ignores this at his/her peril unless he/she is lucky enough to only perform for people who thorugh either natural inclination or behavioral modification are only capable of processing musical information through their ears. But otherwise, if the object is to communicate, then by all means, communicate by all means.
In other words, sometime the "showmanship" is actually a part - to one degree or another - of the music, and looking at it as something "apart" is a mistake.
Sometimes.