PSU's are one of those components in a PC I never skimp on, I used to when people asked me to build a 'gaming' rig for £400 but I warned them then and there that the first thing to go will be the PSU, if they want to fork over a bit more for a better PSU, I will, but it's their choice, but of course gaming first, PSU second.... So far 6 out of 7 PC's I’ve built with cheap PSU's have all died due to a bad PSU, all within or shortly after a year.
Corsair PSU's.... when I first looked into them a few years back, I was seriously sceptical, they seemed like yet another memory company expanding its market through brand power alone. Now, I can't fault them at all, built 6 systems with their PSU's inside, not one has failed, the oldest being 3 years. The same can't be said for OCZ, but that’s another matter.
This AX series looks like it runs on a dual 12v rail system, looking at the picture above, its split down the middle almost. Unless of course it's taken from its server inspired nature and has a single 12v rail with load balance, but just guessing.
The modular motherboard cable actually does have a use... all be it a very niche use... Quad SLI/Crossfire or EVGA's crazy super wide dual socket motherboard with 7 PCI-E slots. If you want maximum stability with a fully kitted out board, you'd need 2 PSU's. That means 2 lots of default wiring to contend with, even though only one needs to be attached to the MB. With the above, if they provide a shorting pin, you don't need to handle all the extra wiring.