If they had put the disc mounts on the chainstay then fitting racks and mudguards would be so much easier, not to mention 2 eyelets.
We have seen designs like these and spoken to customers with them, when using a cable disc brake the cable tends to get full of rubbish because of the angle.
The single eyelet isn't a mistake or accident. We use this system because with the disc mount the only carriers that clear the caliper are Thorn or Tubus ones, both of which have provision for mudguard fittings on the rack giving you a stiffer mudguard and keeping the stays out of the way of anti-sway catches and the like. This is a proven design on the Mercury that we have run for several years.
The frame comes with the adapters to fit V/Canti brake bosses on the lugs provided on the seat stays if disc's arn't your cuppa tea.
Hope this helps explain, Dave