It's all about projection...
>>"If the drummer is sat half a foot from the thing and can't hear it then the audience certainly won't either so why does it matter when he wallops the damned thing or indeed if he does at all?"
Drums are generally designed to project the sound. That is, project the sound away from the drummer. Not a great situation for a drummer on a stage surrounded by other musicians. ;)
Also, when you're sat behind an acoustic kit amongst the general swirling mush of white noise that makes up pretty much all you can hear of the mix of over-the-top guitar amps, bass, keyboards, vocals et al and of course your own playing it's nigh on impossible to discern any kind of definition in the low end frequencies. At most you're likely to hear your hi-hats, snare and cymbals so actually getting some form of feedback from the kick can be important to know you're on the pulse with your right (or left if you're a leftie, or both if you're a metal drummer) foot.
What makes the kick different to the other drums/cymbals is that you're not playing against it with a stick in your hand, but rather with a beater (possibly soft felt) attached to a sprung pedal, and the bass drum head has a lot more 'give' due to its size and lower tuning, so it's that bit harder to discern the actual impact of the beater and/or velocity of the strike. By playing the kick pedal in different ways you can change the character of the resultant note - if you let the pedal rebound quickly from the head you create a much longer more resonant tone, but if you 'bury the beater' (ie: stomp on it with your foot and press it into the head, holding it there after the strike) you can create a much more muffled dry tone, and the harder you bury the beater, the higher the pitch of that tone. If you're playing your kick dynamically like that it helps to have some kind of monitoring.
But then, as a drummer I wouldn't expect anyone to understand the musical nuances of the percussive area of the stage, apart from my rhythmical brother on the bass there :D
But I'll leave you with another 'drummer joke'...
Why are so many drummer jokes one liners?
So that the guitarist can understand them!