I upgraded a friend's system recently and used a WD SATA hard drive that had both standard and SATA power connectors on it. Since his PSU didn't have any SATA power connectors (and the drive was OEM, so no adapters), I used the stardard Molex power connector and everything worked fine.
Later, he got a new case and PSU, and I switched the drive to a SATA power connection. Basically, it appears to be either/or, not both at the same time 🙂