To defend my own actions here a bit; Because one would like to sit close to the front, and have the seat type of choice (e.g. isle). When flying across Europe, on a 4-leg roundtrip, the chances of getting isle seats on all legs is pretty much zero. I'd say the chances of getting isle seats on 2 out of the 4 legs is already rather low.

So one can either spend 15 euro (per leg) extra to book a classic fare, or, risk it with a light/basic fare - and hope that the pre-allocated seats are generally okay, so that at ± 20 euro per seat selection (for the flights where the airline sticks you in the middle or window) you still end up a little cheaper overall.