I normally use 6 bolt rotors but do have some splined ones as well. When I ordered the Hunt wheels with the Mason frameset they were splined hubs. The rotors that I had from another bike were 6 bolt so required a pair of convertors. There are loads of different manufacturers producing adaptors of different designs but all achieve the same thing. I opted for Shimano as I knew the quality should be spot on. They are simple enough to fit onto the splined hub then bolt your 6 bolt rotor to the adaptor. At the end of the day it saves you buying a replacement pair of discs if you have no spare splined rotors. In use I have had no problems to date, never come loose etc.
Manufacturers info - Easily and neatly adapts your 6-bolt rotors to fit on to Centre-Lock hubs. Uses the proven Centre-Lock lockring fixing system for quick and easy installationRRP £14.99 each and I paid £7.81 inc p & p