The rear axle stuff on the disc model sounds like the design and engineering solution to a problem that shouldn't have been there in the first place. The marketing people are presumably OK with locking people into Spesh branded kit... What they've done is come up with something that won't work perfectly with any other road disc wheelset- pretty much every other disc-braked road bike, including the Roubaix Disc, takes 135mm rear hubs of some description. If you do run a third-party wheelset on a Tarmac Disc you need a new derailleur hanger (?!!) and risk shifting problems if you ever cross big/big gears (and though people shouldn't, people do...).
So: If you have a Tarmac Disc and another disc bike, even another Spesh, you won't be able to share wheelsets unless you have time and the necessary parts. No quick swapping of your really nice race wheels for the weekend, or because you've punctured. 3rd parties will have to do specific Tarmac components if they want them to work properly on these frames- and they probably won't. If/when the UCI allows disc brakes for racing, that's yet another headache for neutral service crews and even Spesh teams (if, as on some events like spring classics, some riders are running different frames).
Storck and Colnago seem to have managed to make race bikes without this palaver......