Often use it on the master when needed – e.g., if you have a snare that jumps out too much
I use them all the time. I love the sound of FG-MU, and the HP feature on them is really basic for me right now. FG-MU in HP Mid-Side mode is great for mastering, in HP Stereo i use it od drum groups, on guitar groups i use HP mode FG-MU - or FG-Red on distorted guitars, as FG-MU brightens them a little too much, and i need something more transparent. Probably usable on some synth groups, but never tried it.