We had a landline at first, mostly for emergency. That cost us about $13-15 a month with fees and taxes. And all we got from it was unwanted sales calls and debt collectors who were looking for someone who had that number before.
For people who don't have children (and even then…) and have a cell phone, I don't really see a good reason to still have a landline at this point.
I know that in France, for example, having a landline doesn't really cost you anything more than having the Internet, so most people have one. But $13 a month for nothing helps cutting that cord.