We've always called our citrus the little "Green Hornet". People rarely have a completely unique name. If people can routinely share a name, it seems alright for cars to share one too.
I suppose the only difference is that one silver Insight is going to be nearly identical to another silver Insight and a unique name helps to give the car it's own identity, if only in the owner's eyes.
Speaking of silver Insights, we still have not settled on a name for the new silver baby. We just keep calling him "silver", which is pretty lame...at this point we have ruled out Skeletor and Sunshine.
