The short answer: it’s a lie. DIN is DIN, there are no real cons.
The long answer:
the middle of the range thing is an old wives tail, that maybe used to have some safety validity at some point.
The number scale on alpine bindings refers to a defined spring release torque (within a tolerance), which is a standard all the manufacturers work to, having been set by the DIN (Deutsches Institut für Normung (German Institute for Standardization)) and the ISO (International Organisation for Standardisation) and is set out in DIN/ISO8061 and tested to the methods in DIN/ISO9462
Due to this standardisation, any binding set to a number on the scale should, assuming all other adjustment of toe height and forward pressure etc. is correct, release at a specific torque, or within the tolerance of it.
How close to the defined torque value bindings release at, at one specific binding release value in a range is something that isn’t talked about by binding manufacturers, likely as it can vary based on all sorts of factors, but will still fall within the tolerance.
It may be that the tested value of a binding release setting in the middle of a range is closer to the defined value in the standard than if it is set at the top or bottom of a range but, it will still be in the tolerance of the standard and so shouldn’t be worried about.
One non-safety thing that would be worth considering when buying bindings is how your release value might change as you age and progress as a skier, as height, weight, age, ability and boot size define the recommended binding release setting. Usually this is only by a few points on the scale, if your feet suddenly get bigger, you grow a lot taller or lose/gain a lot of weight. so being right at the top or bottom of the range of a binding could mean you need a new binding sooner.
One last thing to think about is that binding springs wear through use and so the measured release torque value of the binding can change meaning you may have to adjust the binding to compensate. Which, if you’re in the middle of the range allows for any required adjustment without problem. This is why you should get your bindings tested periodically.