I'd transform the comparison between single-speed vs. multi-speed (derailer) to SINGLE-CHAINLINE (single, fixed, or internal-geared-hub) vs derailed (assumed always multi-speed).
Then, there are ONLY advantages for the single-chainline bike:
- Overall material is thicker. On the other hand, the need to pack a lot of gears in a cassette requires that the cogs are relatively thin, and so the chain, and that means wearing out faster.
- Chainline is usually away from the rear wheel. On the other hand, the lower gears of a derailed bike make the chain get very close to the rear tire and get more dirt (MUCH more dirt in case of a fat-tired knobby mountain bike slowly building up mud during an off-road climb in the granny gear).
- The chain doesn't catch so much road grime, (because) in the other hand, derailed bikes literally rub the chain against cogs in order to shift. Also, the cogs that ARE NOT being used keep exposing their oiled surface to the elements, gathering dust and putting it in contact with the chain as soon as it is shifted to that gear again.
- The chain is always in line, so theoretically there is an even stress on the pin/inner-plate working surface, that's where the wearing happens after all.
- It takes at least double the time, work and cleaning materials to clean a derailer drivetrain compared to single.
- As already said, a single-chainline drivetrain can be enclosed.
- Single-chainline usually won't skip when you install a new chain.
So, I would say that for COMMUTING or for utility use in general, a single-geared or internally-geared bike are excelent choices. I have both setups and don't regret.
I don't regret giving up gears in my fixie because it is lightweight and efficient, and I can go fast if I want, and go up most hills if I want (of course not at high speed then).
And I don't regret throwing some serious money for an internally geared hub, because the original cog lasted for at least 20 thousand kilometers, and I saved A LOT of tinkering with the drivetrain (such as cleaning, adjusting, and replacing parts).
Hope this helps!