On road 4 motion makes absolutely no difference over about 10 mph. Its good for increased traction pulling away on damp roads, once underway 3 tonnes is sufficient to keep it well planted, in its normal state only 10% of power is going to the rear, 10% of even 199hp isn't exactly a huge shove of power from the rear.
In snow its stopping that's the bigger problem not traction. A set of winter tyres on a FWD will get you further than 4WD on normal tyres.
On muddy fields it will have an advantage, but in the last ten years by using a bit of common sense, Ive never managed to get stuck.
If you don't intend off roading its a complete waste of money.
According to the 2014 brochure, a FWD 180DSG weighs in at 2512kg the 4motion version is a porky 2634 kg thats the same as carrying an extra 2 adults all the time. From the same brochure, the fuel consumption is 10% worse, the emissions are 10% higher. It accelerates slower, It costs more to buy, has a lower payload, it seats less people.
Perhaps the question should be why on earth would anyone actually pay more to have 4motion.
Thats my case for the defence for sticking with 2 wheel drive, I will now put on a tin helmet and dive for cover.