Burn out isn't just about the physical exersions (sic), but the mental ones too.
If there was no AER yes, the amount played would even itself out (ie it'd be unlikely you'd get the state of affairs with Jelena, once she was free of AER), but you have to consider how AER can potentially lead to mental scars.
Hingis was not restricted by AER, and she's turned out OK I hear you say?
Well, the simple truth is that Martina and her relationship with tennis approaches savant-like status and you can't expect player to have that kind of mental fortitude.
What happens to the Sonya Jeyaseelan to name but one example? Mental fatigue and injury. The vast majority of players can only dream of getting a career high rank of sub 50. And that's what AER tries to protect.
I know what people mean about 'if you're good enough, then you're old enough'. It was great to see Jennifer just totally whipping Martina's arse. Martina Navratilova, that is. But you can still have that with AER, it just safeguards the players against the weariness (... and young girls, they do get weary
) of the Tour.