Powered USB-type risers do not have the input PCIe power pins connected, they have an external power port on the riser itself, usually a SATA or MOLEX connector. This way, both riser and card get all power feeding directly from the PSU, and nothing out of the motherboard.

These risers kill 2 birds with one stone:
You're not using high power over a flat ribbon cable - which is a serious fire hazard btw -, and you're not using power off the motherboard PCIe slots, so you can use whatever motherboard you want, and not just the specialty flavors. In both aspects, it is much better (and safer) to have all power drawn straight off the PSU.

Motherboards with extra PCIe power are a waste in my mind, since you cannot fill-up all PCIe slots without using risers. And as soon as you need to use risers, go with powered ones, or go home.