First of all, what is a knee roll? A knee roll is a raised section on the front edge of a cushion. They are used predominantly in caravans but we do also fined them in some boat cushions. They tend to be about 6"/150mm wide and about 1"/25mm tall and will have a rounded profile.
Just what is the purpose of the knee roll on a caravan cushion you may ask yourself? The idea is that the cushion gives you more support just above the knee joint whilst you are in a sitting position.
However if you have to make those cushions in to a bed to sleep on then you curse the knee roll as it always seems to be in the most inconvenient part of the bed.
You have to ask yourself, do you really need the knee roll? If your existing cushions have a knee roll and you are going to just replace the foam inners then, unless you are going to have the covers altered you will have to continue with a knee roll on the foam. If however you are designing and making cushion covers from new then you may well prefer to have flat cushions as these will also make in to a nice flat bed at night.
Are they then just a fad? Yes and no, certainly they do give more support but we manage to sit on many other types of seat without the need for a knee roll, so we'd recommend that if your cushions also have to make in to a bed then just choose flat cushions.