If they are less than half full in most cases, they will be more than half full in some. And who says that two or more pax traveling together absolutely want to sit right next to each other on a flight that's far from full?
In any case, the point here is that when it comes to separation in the cabin, KLM has chosen to put the interests of the cabin crew above those of the passengers. It cannot be ruled out that some pax who would much rather not sit right next to somebody else will be forced to do so just to keep the aisle seats free. Other airlines have made a different choice.
Johan