I skimmed this thread before posting and I did not notice three very important issues - weather, road surfaces, and luggage.
If you live in SoCal where the weather is mostly perfect and your route to work is glass smooth I don't see how there is any other choice but a road bike unless you have to haul more than a small backpack full of clothes, lunch, tools, flat fixins, etc. My road bike allows me to keep up with traffic more often, get off the roads faster, and expend less effort. This is mostly due to thin, high pressure tires.
If you live somewhere that does not have perfect road surfaces, you often ride after dark, or you just can't get comfy on road bike geometry, then you have to start widening the tires and relaxing the frame geometry. Wider tires roll slower but are more cushioning, are less likely to find a crack to get stuck in, and are more flat resistant. You are far less likely to take a spill on a bike with fat tires because you hit something after dark. Potholes often look like grease stains when illuminated by one bike headlight. Potholes disappear when full of rain water.
Really, you need two bikes. One for near perfect days and surfaces, and one for dark, snowy, poor surface rides. My fat tire bike is faster under bad conditions because I am not tip-toeing around every pile of gravel, pothole, crack, wet leaves, manhole covers, trolley tracks, etc. I take a straight line and blast along.
I am a roadie at heart. I have a full blown road bike for spring/summer/fall rain or shine - daylight only. For winter I switch to my touring bike with 26x2" street tires, panniers, fenders, lights, and so on. My touring bike is set up EXACTLY like my road bike - same handlebar/saddle/pedal relationship - because like I said, I am a roadie and most comfortable on road bike geometry.
Skinny tire road bike - fair weather / good surface.
Fat tire road bike - bad weather / dark / poor surfaces.
If the world was perfect I would get rid of all my bikes except my road bike. In reality, I would keep my touring bike if I could only have one bike.
Hope this helps a bit.