The reason you need a SIM, is because the 4G technology Verizon is rolling out is LTE, which is a GSM variant that requires a SIM card. As long as the card gets a 4G signal it will use the SIM card and establish a GSM connection. If it looses the LTE signal, it will failover to 3G, which is CDMA. And it doesn't do it gracefully either. Any active connections you have open (downloads, VPN, etc) will drop.
The reason you need a SIM, is because the 4G technology Verizon is rolling out is LTE, which is a GSM variant that requires a SIM card. As long as the card gets a 4G signal it will use the SIM card and establish a GSM connection. If it looses the LTE signal, it will failover to 3G, which is CDMA. And it doesn't do it gracefully either. Any active connections you have open (downloads, VPN, etc) will drop.