Lots of people (in all the economic brackets) keep multiple phones numbers in the develloping world..
Some because it's usually cheaper to call within a carrier so they use a certain line to call certain numbers.
Some to be reached more easily (for the same reason as above).
Some because there's no number portability and they want to keep their old number while they want to use a new one for whatever reason.
Some because networks can be unreliable.
Some because they'll give separate numbers to separate people (Pro/Personnal , wife/mistress, anybody/VIP).
And since most people are on prepaid, there's little to no cost to having multiples lines.
I'm not really privy to the psychy of someone who uses a dual SIM, I doubt most readers on Hackernews use one. To me, I visualize a Chinese business man who wants separate numbers to separate his official (married) life and his gray (mistress) life. But this is just a stereotype I developed to deal with going to the provinces.
I'm not sure why people would use it in a high-end phone, but on low-end phones people use multiple sims as a means to save money. Especially considering a lot of developing countries have prepaid schemes. So you'd have one sim for data and another for voice.
Though, thinking about it, maybe people still use multiple sims in higher-end phones for the same reason?
It's supper convenient to have a backup data plan just by switching to your second SIM on a dual-SIM phone, as I do regularly.
It's definitely a not mainstream feature so it falls flat in the high-end flagship phones. Also carriers won't bundle those, so it's really more of a market thing.
I bet many would use them if normal phones supported them.
A common use case is a local SIM card when travelling or commuting to a different country. Even people who don't travel a lot for work will typically travel semi-frequently in other countries in Europe, for family or vacations.
But maybe not in the US...