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.
You're right, but it does feature in some higher-end phones http://www.phonearena.com/phones/Lenovo-Vibe-Z2_id8895