At least in a 387 it is possible to sit in the seat as designed, which whilst not exactly comfortable does encourage a good posture - those who do a lot of gym work will notice this, but it does mean having to sit up very tall, which isn’t what one always wants to do having had a day at work and perhaps wanting to chill out a bit on the train.
However on a 700, it is much more awkward to sit properly because the seat is too close to the side of the train, and in particular because of the ducting in the foot space. So in a 700 you end up with either the legs or, worse, the whole body, as a funny angle, which is most uncomfortable. First isn’t so bad because of the extra spacing, though the ducting is still a bit intrusive.