They told a friend the same thing, it is of course complete nonsense. They also posted on the old C+ forum that the reasons for their using the sliding dropouts was EBBs make it harder to change a wheel! With the cables and chain in place it'd be a very quick job to move the wheel over. All the adjustment will be on the bit’s that stay with the frame. Though you might need to get fast at changing tyres. I know of two people using their Rohloff in different bikes, one Raven/Tandem the other Raven Sport/Custom MTB.
It's a pity One-One no longer make the sliding dropout model, would have been the perfect answer. The £120 sounds a bit dear and unless that includes cable stops and a re-spray it's going to look messy. Might be worth a chat with framebuilder Lee Cooper, he rides and builds Rohloff bikes;
http://l.a.cycles1.mysite.wanadoo-members.co.uk/index.jhtmlAre there any affordable Rohloff specific MTB frames available? I know Kinetics used to sell the Rotor ones, but not anymore. It could be worth waiting until the details of the new Thorn MTB are released.