I can see two reasons, lower price and slightly lower weight, and then there is also an aesthetic reason 
. Why do you think the ex box is so much better, for someone not using disc brakes?
The cost difference is small in percentage terms, as is the weight difference. I'm not sure I understand the aesthetic reasons. I think the photos on Thorn's website might be misleading, making the Nomad look like a rugged dirt bike versus the Tour as an elegant paved road bike. Especially that vile Tonka yellow color. If you get the Nomad in black (which I did) and fit it with fenders, and maybe use medium instead of wide tires, and use some touch-up paint on some of the logos (these appear to be far more obnoxious on the Nomad than the Tour), then it can look very much like an elegant paved road bike.
If you read the Rohloff manual, there is a striking difference in the maintenance procedures for the internal versus external gear mech, with external being much simpler, and the external is more protected from dirt. The whole reason for paying up for the Rohloff hub, for me, is easier maintenance under dusty conditions. By contrast, the easier shifting with Rohloff is of little importance to me, since I didn't find derailleur shifting to be difficult, as long as the derailleurs were working properly.
I'm using rim brakes on both wheels currently, but I plan to ride this Nomad for 20+ years, and my opinions might change down the road. So having the disc brake option is worth the cost and weight penalty to me. Mechanical disk brake on the rear, rim brake on the front seems like a very good combo, and I would hate to feel myself limited by not having the option to at least try this combo.
The external gear box is the main advantage of the Nomad, but I think it might also have room for wider tires (60-559 vs 50-559) though I'm not sure about this. If this is true, then is definitely a reason to go with the Nomad, IMO. You don't have to go off to the Himalayas or the depths of Africa or whatever to encounter rugged roads. Spain, for example, where I have plenty of hiking experience, has plenty of rugged dirt roads that would be ideal for bike touring. I would imagine there are plenty of rugged dirt roads right outside of London. Not every road is paved and anytime you are dealing with unpaved roads, there is the potential for the road being extremely rugged. So if someone is going to buy an expensive touring bike, I think they should plan for rugged conditions at some point, even if they plan to mostly stick to paved roads, as long as planning for rugged roads doesn't ruin the bike's performance on paved roads. The loss of performance from going from Tour to Nomad, if any, is minor.
Now that Thorn has both the Nomad and Nomad X, the Tour makes even less sense, if what I said about the Tour only taking 50-559 tires is true. I'm not surprised by this proliferation of models. Consumers like to have choices and Thorn has to accommodate that. But if you can only afford to buy one touring bike in the £2000 range, then I'd recommend getting the very best of the lot, which is the Nomad, IMO, rather than trying to save £150 by going with the Tour.
