MM design objectives - idle bias levels

These message boards are extremely useful! Many thanks to Mike ( so far). Now I've started to get the idea about bias levels ( I have an RD50 with fixed bias, apparently) it's clear that MM amps are biased very cold( ie low idle current) compared to other amps. I get it that V+ is pretty high, although in the RD it's only 520 after you subtract the cathode voltage of 60, but even so these amps seem to run an idle bias of only a few ma across the 6.9 ohm resistor in this amp( unless my VOM is reading wrong!)
Do any of you experts know what was the design objective-apart from tube life - in biasing the amps so cold? I would have thought a 6l6 running at 500v or so could take maybe up to 20 ma without danger ( ie 70% of dissipation)