You could do that with regular SIMs. It would be a PITA to switch SIMs over manually.

Of course you'd have to carry a hefty Verizon postpaid bill.

The US carriers these days, their game seems to be to try to get customers to add multiple lines. At least that's where the promotions are. So in addition to trying to lure new subscribers over, they want to get existing subscribers to pay more for additional lines, which they may not need.