Solution: Only ask for the amount of vocal mics that you really need.
Problem: Having a lot of mics on stage that aren't regularly used creates a lot of extra noise out front for the audience. Depending on how shallow the stage is and the amount of stage volume (ambient noise from the drums and amps, etc), this can be a real problem for the sound engineer. It can make your band sound "sloppy", can increase your overall volume out front and will be more difficult to get vocal clarity to the audience. Loud is NOT always good. Trimming down just a few unused mics on stage can really clean up the sound out front and let you concentrate on what's important - your playing!
Background: Sometimes certain band members feel left out or less important if they don't have a vocal mic in front of them. Other times they simply want a mic to interact with the audience, even if they don't sing. Other bands don't do harmonies but are heavy on "gang vocals", so they want the extra mics. Each and every application is different and should be given its own inquiry as to what the best solution is.
Mitigation: Think about each person who usually gets a mic and the amount that they contribute vocally to the performance. One rule of thumb may be if you don't practice actual harmony lines during auditions/rehearsals, then maybe that person doesn't really need a mic during the show. If your band is routinely performing 3 and 4 part harmonies, then maybe the extra mics are needed. If you have a song or two that is sung by someone other than the lead vocalist, maybe sharing a mic makes more sense, or simply telling the sound engineer that he/she can mute the extra mic until that song appears in the set list. There are almost always multiple solutions to the problem.
Working with your engineer to establish the best sound out front should be the goal.