Depends on your perspective. Yes, it's less efficient for me (individually) because I'm subsidizing suburbs and paying more taxes.

But, I was talking about the city, society, or the greater good. Whatever you call it. Higher density is more efficient and uses less resources per capita. Period. Same reason private planes are so bad for the environment.

That might be true that higher density is more efficient, but efficiency isn't a sufficient metric for living overall. Putting everyone in camps would be more efficient too.

But do you really pay for suburbs? As I said, the cost intensive positions scale with the number of people. Schools, utilities, ...