I see similar problems with people adding lights to older installations. It's not so many years ago that the typical British house was wired with just a single overhead light fixture in each room, and maybe one or fancy ornamental wall lights in the living room. These days, people want to add all sorts of extra mood lighting, and they usually just tap into whatever lighting circuit is nearest.
I've even seen a whole load of this stuff added to crumbling old 1930s wiring. I haven't seen wiring this old feeding wall receptacles in a long time, but it's survived in some older houses on the lighting circuits.
E.g. here it would be normal to have half a circuit upstairs and the other downstairs so that when a fuse/MCB trips perhaps one room upstairs and one downstairs will go dark rather than an entire floor.
Unfortunately, that seems to be the exception rather than the rule here. Many smaller houses used to have just a single 5A lighting circuit which coped adequately with the "one light per room" type of system.
These days, it's normal to provide at least two lighting circuits, but in the average two-story house the most common arrangement seems to be one circuit for upstairs, one for downstairs.
I agree that the split method is far better, and I try to wire my lighting circuits that way.
