I think that it's not exactly like you wrote. Additional power for card is required (or not) depending on what devices you want to use with card. All devices with their own power supply (scanners, printers etc) will work without molex. If you want to use bus powered devices (a lot of), card might need this molex. It must have something to do with overall PCIe power draw, which is limited to about 300W for all (four in your case) PCIe slots.