Good lighting isn't as pedantic or academic or simplistic as using hard 3-point lighting -- that's only meant as a starting point for discussing what the light is doing in the scene. Modern cinematography is more about simulating or recreating natural light but in a way that creates the right mood, or makes the actors look a certain way (generally more attractive but not always) or at least, shows us their faces so we can read the performance.
Softer lighting is generally harder to control, which is why it takes some skill to use it AND create mood and contrast. Usually this involves careful use of flagging.
You could, for example, bounce a light off of the ceiling but within a black cloth skirt you've taped around to the ceiling to keep the bounced light off of the walls.
Two people sitting on a couch can be lit many different ways -- for example, an overhead Chinese Lantern flagged off of the walls. Or crosslit from the sides with soft back-edge lights as if from table lamps.
Look, if you're intent on just shining spotlights at actors in small rooms, yes, you'll probably find 150 watt units are bright enough in many cases. I can't believe you haven't noticed though that that's not how most movies are lit though! Low-key soft light with some contrast through flagging or controlled fall-off is more or less the modern style. Hard spots for actors at night, indoors, are generally only motivated in rooms with track lighting. And often those aren't proving key, back and fill.
As for why not just use a lower wattage light than diffusing it, the answer should be obvious if you think about it: a soft light LOOKS DIFFERENT than a hard light. If you want a softer light on a face, then simply going to a lower wattage hard light is not going to do the trick, will it? If you don't understand how a face looks different in a soft key or a hard key, you need to watch movies more carefully and look to see how defined the shadow pattern is on a face.
As for BRIGHTNESS, that's a question of exposure, isn't it? After all, the sun is an incredibly bright light source, so why isn't everything you shoot out in the sunlight look horribly overexposed? Because you expose for the sunlight so that the brightness looks normal. You can even underexpose sunlight to make it look like moonlight. The only problem with using too bright a light indoors is that while you can expose for the light so that the subject is normal in brightness, you may now have too much depth of field from stopping down the lens, or you may be overpowering all the natural light in the scene so it looks darker in comparison.
