I did a no-sparge last week and hit 65.7% and 12.6 plato. Wasn't aiming for better either. The number 75% has been bandied about by experts like Jamil Z. and John Palmer as a decent number to shoot for in extraction. When I batch-sparge my BE is set at 75% and for an average strength beer this is no problem.
When I no-sparge, I get 70-75% efficiency. If you work through the numbers you'll see that's what's expected after complete conversion. I know it's popular to scoff at high efficiency numbers, but it's not imaginary, it's simple physics. Kai has gone through a lot of trouble to show how these numbers are derived, so I won't try repeating it here.
If JZ and JP are getting 75% after sparging, it's due to the inefficiency of their conversion. Without knowing their actual conversion efficiency, it's impossible to say how inefficient, but it's clear that they must be getting incomplete conversion if they only get 75% efficiency. Unless their sparge systems are unusually inefficiently designed, which I doubt.
An average gravity beer, using a standard batch sparge protocol, has aaround 85-90% theoretical lauter efficiency. With 100% conversion and low dead volume in your tun, it's simple math that leads to the commonly seen efficiencies around 85%. 75% mash efficiency is, at best, 85% lauter efficiency of 88% conversion (0.85 x 88 = 75). At worst, since those brewers are probably fly spargers, they might be getting even lower conversion and only hitting 75% by pushing their lauter efficiency. If that's the case, it may be understandable why they would think
that higher than 75% would be undesirable.
However, it's too complicated an issue to give such a trite and arbitrary answer as "75%", and it's meaningless if you don't consider conversion efficiency.