Fitbit, the app that so many joggers can't do without, is expected to price its initial public offering on June 17, much to the joy of the investment bankers at Morgan Stanley, Bank of America Merrill Lynch and other financial firms helping to sell the deal to investors. The interesting thing is how rare IPOs like this are at a time when "unicorns" - startups valued at $1 billion or more - are the talk of Wall Street.