Never understood why any mic needs to be live when not in use. Every mic I use on a gig will be equipped with an on/off switch, and it's expected to be used. My own vox mic is only ever on when I sing. (I don't want the audience hearing my asides!)
As a former soundman, I would always pull a mic down if it wasn't being used - it saves any possibility of feedback, someone knocking the stand with the resulting clunk, etc.Especially if I knew there was no switch, or the person using it couldn't/wouldn't use its switch. BUT - I needed to be aware that a certain mic was about to be used, so to fade it up.
'That's not how it works' is a pretty lame reasoning - explanations please, we're not idiots.Get used to being in charge of your own mic, which can be left faded up, but turned off till needed. And don't come up with 'my sm58 doesn't have a switch'. Get a switched XLR soldered onto the lead, so it does! Try Neutrik.
Lead question answered - feedback potential, noises from stage floor, messing up sound balance with bleed from sources close to open mics. Close them!