I guess I'll go ahead and chime in my $.02....
ATF, years back, IIRC, was basically whale oil. Remember reading in school about the eskimo's using seal fat for lamp oil/candles? Same difference. (plus at the same time frame ATF was considered a skin-ailment cure-all)
Nowdays, ATF is NOT anything like it's older cousins. However, facts are facts, ATF is a high-detergent, low-viscosity lubricant. Whenever we get a vehicle in the shop that was neglected, crusty carbon buildup in the valve cover, thick nasty oil, etc., we'll drain the oil, leaving the filter, and refill to full with merc/dex atf. We crank it up, let it reach operating temp (ON JACKSTANDS, NOT DRIVING) and then drain it out and do a full oil change, with good quality oil (usu. castrol 20w-50, but that's in the warm south). It does what we want, gives everything a decent cleaning, helps re-animate some of the super-thick oil deposits, even helps break up the carbon, so that the new filter can do it's job. Due to it's low viscosity, I recommend that NOONE put it in their engine and drive around like that.
For fuel systems, again repeating myself, it's a low viscosity high-detergent lubricant. On older gasser vehicles, a quart or two added to a full fill-up at the pump would help quiet the noisy electro-fuel pumps, give the tank a bit of a cleaning, lube up the carb's internals, help de-funk the valves, etc. The same can be done on gassers by using two-stroke oil (outboard oil). I've done as high as 40:1 ratio w/o any smoke.
For diesels, It basically has the same effect. I think someone has already stated, that a diesel could run off of just strait ATF, that's true. Cost keeps me from doing that, and depending where you are in the world, the color isn't a good thing either. Got hit one night crossing the LA/TX line by Texas DOT, and they swabbed my rabbit (which I didn't think they could do, because my vehicle wasn't a commercial rig with a DOT registration) and I had about two gallons of recycled atf in the tank, so the swab came up red, and they accused me of tax-evasion, using off-road (agricultural/marine) diesel. Needless to say, they didn't even write me a ticket, just hassled me real heavy, plus I had another couple of gallon jugs of old atf in the trunk.
As a every-tank fill-up additive, I think it's a waste of money. But every couple of tanks, sure, throw a quart or two in before you fill it up with fuel. It mixes very well with both diesel and gasoline, and even at over two gallons mixed with a full tank of diesel, I've never produced exhaust smoke. However, when poured directly down the carburator of a running v8, it will mosquito-fog and entire county!!