But why there should be one for a high-end motherboard?
It's not just a "high-end" motherboard. As you've noticed from MSIs own marketing babble, it's a product aimed at professionals and overclockers.
For overclocking PS/2 is mandatory, cause if you do Dice or LN2, you have to disable as many peripherals as possible to achieve best stability. This includes PCH USB hubs, third-party USB hubs (like that AsMedia IC), unused SATA etc.
For professionals it usually boils down to ease and cost of integration into existing infrastructure. So, if it's used for rendering, it might be connected to a KVM switch which with a legacy interface like VGA/DVI+PS/2.
For enthusiasts PS/2 is also a desired option, because USB can't offer a true NKRO, and software implementations usually suffer from increased latency: even though OEMs will shove a bunch of bullshit marketing numbers at you, like "this KB has a maximum 1000Hz polling rate", "Supports NKRO" etc., which means jack shit if your KB can't reach anywhere near that max with NKRO enabled. Whole different story with gaming mice, where raw throughput is actually more important (with some caveats, of course).
It's not super-duper critical and most people can get by without PS/2, but it should be much higher on the priority list than having a CLR_CMOS and FLASHBACK buttons on the rear IO panel and not some other spot, or more important than having a third-party USB controller for no reason at all, or cause they were too lazy to do a breakout directly from AMD chipset (x399 has two built-in Gen2 lanes and 14 Gen1 lanes).
People are "yay, finally we got rid of DVI", and in the meantime "there must be a PS/2 connector for grandpa's old keyboard"
I was never a big fan of this decision either.
Let me give you a few examples from my every day life...
One of my usuals (IT company w/ 40+ employees and even more PCs) has a bunch of older Dell, BenQ and Samsung monitors, which only include VGA and DVI ports. No one wants to spend a ton of money on upgrading perfectly functional and more than adequate monitors until they die or start malfunctioning. This means that every time we buy a new PC, we have to invest into additional stuff like passive HDMI-DVI cables or active transceivers.
My other customer was also furious, when I told him that we need HDMI-VGA or DP-DVI adapters, cause his old but once very expensive KVM switch, which was doing just fine for the past 5 years, cannot be connected to those 6 Skylake workstations which we needed to prep for departure to Huawei HQ in the next 4-5 hours. They did have PS/2 ports, thankfully.
So, I had to make a quick run across the city and spend an extra $100 and 2 hours of work time, cause "old interfaces", and "compatibility" for some reason are the least prioritized aspects nowadays, even in enterprise market segment.
