All things being equal (which is damn near impossible between the two main feds), you can squat more with a mono cos you are not having to deal with the walk out. It may not seem like much, but when you are using your max weight, even a slightly soft unrack can destroy a squat, not to mention walking back with it and getting your feet placed right. There can be a huge amount of energy leakage (if I can use that term) with a sub standard unrack and walk out.
Lets face it, if there was no advantage, they would not be used.
I have heard, not sure if true, that the first monolift was made after a guy in a comp had the spotters push/tilt the stands forward so that he did not have to walk out. Obviously he knew that he would squat more without the walk out.