A lot of the guys I have seen running "tactical" armor carriers don"t see the need for a handmic. So the radio has to be removed from the pouch to be used. There is a Motorola at 10:00 abover her belt line. I just lost a trainee who could not get it through her head that using her handmic in the unit with the unit radio on caused painful feedback for every one on the channel. I run an earpiece so that I can keep my handheld on at a useable volume without feedback when I use the unit radio and switch the unit radio to a neighboring agency's channel to monitor while still being on my channel (not that that also hasn't caused problems). I have seasoned 25+ year officers who will either run their handhelds and turn the volume on the unit radio off, or else turn their handheld down/off when in the unit. For a long time I was the foot pursuit king and learned the earpiece in and radio at a useable volume means I didn't have to either turn on my radio and wait for it to boot up or turn it up early in the foot chase so I could coordinate containment. By the same token, when I jump back in the car and give chase I don't get feedback.
I don't know if it is still the case, but due to our wide open spaces (old timers used to speak of backup being literally hours away) State Police used to have a radio repeater in the trunk of every unit. I know that in the past it has caused problems for them on large scenes.
pat