You won't get any arguement here about the need part. It isn't only Disney that calls it an electric CONVENIENCE vehicle though. I had heard the term long before I got involved in disability info for WDW. I expect some marketer thought it was a good, non-threatening name for people who needed one, but didn't want to
admit they needed one.
For many people, it is the only way that they are able to go to a place like WDW. For every person who uses one, there is probably someone who is walking around in pain, but doesn't want to admit they need one. And, did you see the Michael jackson interview? He had one to use for
fun late at night in the hallways of a Las Vegas hotel. That doesn't exactly give people the right information about
ecv users.
Anyway, for the CMs who told you to park the ecv and get in line, it's possible that some of those lines were not ecv accessible and the CMs were just not tactful in telling you that information. Many, but not all lines are accessible. (Although it's also possible that they were just insensitive).
Here's a
link to the DIS page that lists attractions/lines where the lines are totally accessible (Mainstream Lines).