I first noticed a few years ago that as more items are getting connected, the management of that connection becomes a burden. You've got companies putting SIM cards in watches now which seems unnecessary today, but soon we'll look back at it and wonder how we ever lived without it. There are SIM cards in phones of course, but also tablets, cars, home security systems, and more.
Do you manage each of these individually with their own account? Maybe, but probably not. It's a bit scary to think that if the devices are all on one account, a hacker could obtain power over all of the accounts that connect and secure your entire life; but realistically this is only one entry on a long list of horrible things that could happen if you don't take security seriously.
Unfortunately, the billing methods haven't really caught up to our demands on the carriers. For example, I want a SIM card in my iPad for travel, but I don't travel often so I don't enable it most of the time. If I put the iPad on my Verizon account with my phone, I share my phone's data and I have to pay $10/month for something I rarely use. As a result, I have a separate account for my iPad. Then you've got family sharing issues where there's a single primary account holder, how would one of their children add a device to that plan without being a burden on the parent? They can get authorized on the account, but even then the power is limited.
The solution isn't crystal clear yet. Though eventually carriers will have to take a big step forward with much more clear online accounts with easy permission controls so that each user can add/remove devices at-will. The billing needs to support flexibility and non-permanent device additions – even though the carriers have a large vested interest in you adding your iPad to the account then never using the data plan on it.
Another option is for each endpoint to be it's own account with the carrier and carrier billing being invisible to the user. For example, imagine a next generation Nest thermostat with a SIM built in (to guarantee remote control capability in either a vacation home with no wifi or when wifi is down at home). Perhaps I can just pay Nest $25/year for that capability (it is a negligible volume of data, after all). They handle the carriers on their end and the user doesn't have to worry about it.
This is only going to get worse in the short term, and I don't think the carriers are properly motivated to really solve it. People are complacent by their nature, so if you can lure them in, the odds of them ever proactively seeking change is shockingly low.