While true, it also waste's power overtime and is not as efficient or reliable. In the long run you may actually lose money.
On a single rail system you have power being pulled from that rail alone and all power needed is being used.
On a multi rail system all the power being allocated is not used. IE: 36 amps to CPU on rail 1 and 2 but CPU is only calling for 22 amps, the left over amps can not be used by the GPU, it must be pulled off another rail. Such a dirty system.
A well made single rail design will outperform any multi rail design.
Because of today's safety standards (20 amps per wire), most multi rail PSU's are actually single rail units with split 12v current limit circuitry.
If you do go with a (fake) multi rail system, make sure it has Active PFC and EVERY "rail" is OCP protected (current limit protection [over-current] ).
If you go single rail, make sure it is a quality piece.
Either way you ARE buying a single rail system. If your buy has more than one rail you best make sure it has OCP to each or you just bought a pile of junk.