I set it low because it's easier for a customer to raise action than it is to lower it. For example, people that are getting new Jems and are having problems lowering the action. If they got them extremely low and playable in the first place, then raising wouldn't require any major adjustment (or fret job) at all.
I rarely set it lower than .75mm because then you get into very procarious territory (buzzing,playability issues,etc). Especially in my region where the temperture and humidity can vary greatly on a daily basis. It becomes a game where my guitar laughs at me when I adjust it on a 50 degree 20% humidity day and the next is 82 degrees and 95% humidty. Plus, since Maryland is at the confluence of the gulf stream, northern mountain effect and the southern upwind, I live in one of the most radical weather enviroments in the US.
Low action is not harder tension wise...that's dependent on the string gauge. But it is more difficult to get a good dig. Which makes it more difficult to push up on for a bend. I've never had action affect my sustain unless as YngVaiTriani stated "it buzzes"
I like my personal action @ 1.1-1.2mm depending on guitar. I set low at construction so I know the fretboard is ready for anything. *On my 8 and 9 strings, some people will play these in the vain of
Stanley Jordan, Charlie Parker, etc. Thus low string action is necessary. Thanks for the input, keep it coming!