The 7th i don't really understand especially here in the states where over 80 mph and you get a ticket. C mode kind of stinks on the freeway as it puts you in 7th and it lowers power. Sport though is pretty good for a 2 litre motor considering you can also get really good mpg at 70 mph in a 4x4
As per nightscar, its fuel economy. I was out for 150 mile trip this weekend in mixed driving, a lot of it at 75-80 and the car returned 28.3 indicated. Impressive. And of course, 7th is just an example. The cars turns ~1250 RPM in 5th at 42-3 and sips fuel in the process. If I want to get out and pass, I just use the paddles. That, after all, is why they are there.
I see the mode thing a bit differently than you. I have no desire to needlessly waste fuel. One of the reasons I bought the GLA was the promise of this engine's efficiency when not called upon and its supreme ability to deliver when tapped. So, for me, C makes some sense as the car does a better job than I can or at least care to in terms of maximizing shift points for fuel economy. If I want to get on it for whatever reason, again, I use the paddles. IMO, if a driver is too lazy to be bothered pulling a paddle to access the performance, than, as I said above, buy something else with a lot of displacement and be prepared to pony up at the pump.
I personally see less value in S. Given how short 1st and 2nd are and how hard the rev limiter is, S makes a modest case for itself in terms of being a bit better at optimizing upshifts from low speed when on maximum attack. But I didnt buy this car for its 0-60 time. I enjoy corners and wanted something in a practical package to tackle them with. If want to drive hard, by definition it requires more attention and involvement. In those instances, I select M and make my own decisions. And in fact even when I'm not interested in pushing it, I mostly use M. I've spent most of my life driving sticks, so I don't find shifting an imposition of any kind, quite the contrary really.
Others may approach things differently and for them I can understand wanting something a little more telepathic or alternately with a lot more low end grunt. But I really like the setup in the GLA45 as it is. 6% more HP might be nice and the gearing might be nice as well, but I'm fine with the package as it sits. Its quite endearing and is rapidly making a case for itself as one of my personal top 5 favorites... and I've owned quite a few cars in my life.