I would opine that, for the most part, it's the casual tourist users who are going to spend their time and money in the gift shop. They're not the ones likely to spend their sailing time on the car deck anyway. Further, I suggest that people who use the cafeteria do so because they're hungry and while no one would mistake the fare for something out of a Michelin star rated restaurant, it can be convenient for travellers. You're stuck on the thing for a couple of hours, why not grab a bite and save time at the other end?
I think we can confidently assume that the likes of KV1, Clint, a bunch of truck drivers, along with a whole bunch of other regular ferry users, aren't going to be suddenly feel coerced into going either to the cafeteria or the gift shop because they have to vacate their vehicles should they be assigned to the lower car deck.
What I find truly astounding is that this thread has reached it's third page!! It's going to be a long off-season....