We don't know that yet. That's one of the questions we have, and it's one that's posed in any of the literature you read about it: what are some of the long-term effects of inhaling the vapour itself? It is a cleaner form of nicotine delivery compared to cigarette smoke, but we don't know if it is really safe.
We'd prefer to err on the side of caution at this point, to make sure that people aren't unduly exposed to things that may be found to be problematic down the road after millions of people have been using it for prolonged periods of time.