This approach can work for some genetic diseases such as blindness based on some cells in the retina or partial blindness. For others this is not really a cure. If you want to cure people with progeria, does curing 20% of the cells really help? Perhaps 100% is not necessary, but it would seem strange to cure only some cells but not others. You'd have a mosaic of cells where some would work and others don't. Cells interact; timing also plays a role in development. I don't really see that aiming for anything but a very high number of cells cured, can work.