one of the hardest parts in starting the mixdown, is getting the sub and kick balanced, so for the purposes of a little nudge in the right direction i think its useful to have that db refrence to aim for. the more you learn, the more you realize you dont need to stick to the numbers in the same way.
subprime, what I think you are referring to, is running a channel at +0DBFS, clipping it even tho the master channel may be much lower, and you are correct, you cant stop it clipping by turning down the master fader, once a signal is clipped, theres no going back, you have to turn it down at the source to solve that problem
RE clipping and hot digital signals - a digital signal doesnt run hot at all, its either clean, or clipped, plugins may simulate a 'hot' signal, but its not really hot in the true sense - a hot signal comes on analog gear only, lets say we have a 0DBFS digital signal, the loudest possible digital signal before it clips, if we run that into an analog mixing desk, it would come thru at +8DB VU, this is a very hot signal, in the digital world it doesnt matter, the signal is calculated mathematically, so its volume level has no effect on the way it runs thru the system, but in an analog system, +8DB VU is generating significant heat in components and changes in the magnetic fields within the hardware, this adds color to the sound, changes the way it behaves, and is what is ment by a 'hot signal' (high volume = high voltage = hot electrical components)