IMHO, it depends on the venue. In the mountains of East Tennessee (USA), audiences expect you to mix-it-up. Our group is very fluid, with different people all the time, and we work out play lists that emphasize and showcase the specific skills of the performers.   
    This is very common in old time and bluegrass music, where different instrumentalists will take the lead, take breaks, harmonize, etc. The same skills come from the singers. The tricky part is to split the set equitably so that each of the performers get equal time, and, at the same time, entertain the audience.
    With instrumentalists' bands here, with no singing, they often seem to be entertaining either themselves or the other musicians, which makes for a small crowd (usually).
    Wilco in USA