Constant light pipe

Light pipes (or sun pipes) are a means of providing sunlight to places it wouldn't otherwise reach. But they're not widely used - why is that?

I suspect it is because they're not a reliable means of illumination - on overcast days they may not provide enough light to work, and at night you'll need another light source anyway. Thus they tend not to be installed.

I propose that for large multiple-occupant buildings a more integrated approach would allow the effective use of sunlight where available, providing a decent reduction in electrical light usage.
A large light collector is installed in the building's roof. Because it's serving many rooms there are economies of scale, and it's probably worth using a sun-tracking device to increase the efficiency.
Inside the building, the light is split into separate streams which are piped to the appropriate rooms.

But here's the clever bit: the light drawn to each room is dependent on the need. There's a light switch (can be a dimmer switch if necessary) in each room, and the amount of light directed there is regulated dependent on that. Furthermore, when the sun is not providing enough light due to weather or being on the wrong side of the planet, electrical lighting is used to boost the supply.
Changing lightbulbs is necessary less often, since the system can accomodate a few broken ones (and they presumably get less use and switch on and off less frequently). Plus it's easier, since they're all in a central location.

If there is a surfeit of sunlight, the surplus could easily be redirected to the underside of a solar-thermal array, providing more hot water.

The building as a whole therefore uses less electricity. To deal with the human nature, there probably ought to be a charge dependent on the amount of light transmitted into the room. The rate be reduced dependent on the proportion of sunlight used, but would never be zero to prevent people demanding maximum illumination the whole time.