I thought it was the other way around. VZW's CDMA phones support GSM so that their customers can use them while traveling internationally, but you're unlikely to find a GSM phone that has CDMA just in case you want to switch to VZW.
There are plenty of cheaper CDMA phones that don't support GSM. Most of the higher end phones support both.
And then there is another scenario like I believe some of the Google Nexus tablets only support LTE on Verizon and won't work at all if there is no 4G on a CDMA network. It will work on 3G/3.5G GSM networks.
