Symmetry and stability are excellent. With a short bellows, need for longer movements, or energetic bellowing, it may be less suitable. Wear of the instrument hardly is a problem but wear of the clothes may be. A method, which combines fairly good symmetry with very good control of the bellows, is illustrated in Figure 4. Just the edge of one end is resting against the knee and the instrument is balanced in this position while the bellows is worked as equally as possible with both arms like you open and close a book.
Some tutors explicitly speak against any twisting and uneven movements of the bellows.
However, as mentioned above, the flexibility versus rigidity of the concertina bellows is a controversial matter and it has such significance that it ought to be the subject of a careful individual choice. For the instrument maker it is controversial too, since making the bellows with considerably more folds comes outside the standard procedure. None of the authors however present any kind of analysis of the idea or their motives for this advice. My guess is that they have preferred the horizontal forearm position as it resembles the position when sitting and that the insecure original way to hold the instrument gets even worse with a lower position. There are some other things to consider too.