I assume the main reason they are banned is because airlines don't want others that do no understand the technology to think that it is okay to light up regular cigarettes.

Otherwise, it's just a little scented water vapor that escapes. I have several friends that use it and in any kind of reasonably well ventilated area it's hard to tell they're even using them.

The smell would be a bit annoying on a plane (depending on the liquid) but otherwise causes no health or safety concerns for fellow passengers.