Plenty of people may use them but that doesn't mean I would. I have very high standards when it comes to psu's, for example Corsair and PC Power & Cooling are pretty much the only brands of psu that I would use.
Multiple rails allows the manufacturer to use several cheap rails instead of one high quality one. In my opinion that plus more usable power (you aren't limited by what connectors are attached to what rail) and no need for load balancing are more than enough reasons to go with with a single 12v rail psu.