You need not blindly trust me over that. You may just connect some a700/850/900 by its USB2.0 link to a computer and watch how fast that 1-bit serial interface drains the cam's battery. That happens because they are high-end cams, and so the interface is operated at the max speed possible due to performance priority in design.
Agorabasta, thanks for confirming my the wisdom of my decision always to download through a card reader. I now never connect any camera direct to a PC to download pictures or video. The mini USB connection point is for firmware upgrades.