There's a difference between adding a switch or receptacle to an existing house, where I'll get power from whatever is nearby unless it needs more capacity, and wiring new construction.. It can take almost as much effort to combine lights and receptacles as to wire them separately. I look at it as a dynamic system, not a static image.
When wiring, for example, the kids' bedroom area, a single 15a circuit is good for three ceiling light/fans, three closet lights, and hallway and bath lights/fans, while a single 20a circuits is enough for three bedrooms and the hallway. The lights are wired from switch to switch from above, and the receptacles are wired from below.
I sometimes get service calls because the bedroom breaker trips while vacuuming, and discover the customer or their home-cleaner is in the habit of having every light in every bedroom on while vacuuming the floors. Lighting and receptacle loads are not the same, not like the old days when lamps were the major plug-in devices.
I also do the same thing in dens, living rooms, and other common areas where there is usually enough lighting load for a lights-only circuit,which is calculable and relatively fixed, and the increased use of home-entertainment electronics and unknown future plug-in equipment suggests a heavier circuit that won't kill the lights.
