I know the following:
- Setting max_usb_current=1 sets the available current over USB to 1.2A (default is 600mA)
- This can help if you have a decent power supply (2A, at least) and need to power something like a small external HDD or something that needs 300+ mA.
- This feature is only available on the B+ and Pi model 2 (at the time of this writing).
But some of the people mentioning the setting warn against setting this value unless you absolutely need to... my question is, why is that? Is something else disabled or changed about the Pi that would increase a risk of damaging the Pi or anything powered over USB, or is it more a general warning to scare off people who don't know what they're doing and might not have a decent (2A+) power supply to the Pi?