As long as CDMA is in play, Verizon holds all the cards. With LTE, and now with VoLTE coming, the question is open. It would be interesting to see what happens when a 4G/LTE sim card on VZW (that is already activated) is put in a Moto X Pure device. My guess is that the LTE would work. But you couldn't make phone calls. And SMS wouldn't work either.
When the 2013 Nexus 7 LTE came out, it was ready to work on Verizon's LTE network without CDMA. However, there was a problem because only CDMA devices could activate a Verizon SIM card. So, you couldn't activate the Nexus 7 on VZW with a new SIM card. But, if you put an activated SIM card into the Nexus 7, the LTE worked fine. About 6 months later, Verizon updated their network activation, and now you can activate sim card using only LTE, and Verizon began selling the Nexus 7 at that point.
The interesting thing will be what will happen with VoLTE. The current Moto X on VZW doesn't have VoLTE enabled, but indications are that it will come in a few months (I'm guessing/hoping with Android L). At that point, you MAY be able to put a Verizon SIM into a Moto X Pure Edition and even make phone calls, although I'm guessing you would have to port over the VZW VoLTE changes, which I would think would be easy since the Pure Edition can be unlocked
Almost makes me want to order one and try it!