Almost never will anyone, including the bride, know every person at the reception well enough to judge beforehand. In fact, in an average sized reception crowd of 100 to 150 people, there may be friends of relatives of the groom in attendance whom hardly anyone will recognize. There is good reason. The guests at your wedding reception will, in all likelihood, be coming from diverse locations, lifestyles, and musical tastes. The primary mission of your wedding reception band should be to provide entertainment that everyone will enjoy.
All brides like to see the dance floor full at all times.
With a good variety band keeping the show well mixed, you will see your guests entering and exiting the dance floor according to their musical interests. Of course, even the best variety band is not going to please everyone all of the time. This brings me to the point in question. You may have attended a friend's reception and noticed that a larger than expected number of guests left early and, as the night went on, the intensity of the party seemed to die rather than build. This might be the result of a so-called variety band that is actually a specialty band trying to play variety.