When you’re recording, you want to capture not just the notes, but the vibe of the performance. For some singers, this means grabbing the microphone and rocking out the way they do on stage. This can cause problems, unfortunately, when recording. First, it’s more difficult to maintain a constant distance between the microphone and the vocalist’s mouth when both are in motion. Second, if a singer likes to wrap his hands around the mic’s pop screen or capsule, that can change the directional and tonal characteristics of the mic. Third, the best way to handle plosives and sibilance is with a pop filter, which needs to be attached to a mic stand. And those are just three reasons not to hold a dynamic mic; many studio condenser, ribbon, and tube mics aren’t meant to be used handheld.
As much as it may put a singer in the mindset of being on stage by holding the mic, in most cases the drawbacks to holding a microphone when recording outweigh the benefits. Best to set up a stand, attach the microphone and a pop filter, and stand at a distance that controls proximity effect.
If your vocalist just can’t handle not singing into a handheld, mic, try setting your studio vocal mic up back a bit further, then give them a handheld mic to rock out with. They’ll get the vibe of singing handheld, but the sound of the stand-mounted mic. You could even record both the handheld and standmounted mics to see which sounds best in the track — in today’s world of endless tracks, there’s no harm in having an extra option!