ev-do is the 'internet' connection used by the phone. Mobile web and other vcast things transfer over the ev-do network. A lot of time this is not needed and just wastes battery power, which is why you may want to disable it.
What kind of battery savings do you note running with EVDO disabled?
Also, I enjoy the Verizon Backup Assistant program and sometimes use video and image texting. Is there a way to determine what services utilize the EVDO signal versus the CDMA signal?