Here's another reason to be wary of Spotify's public offering — the amount of money it brings in for

The paperwork Spotify filed Wednesday to become a public company offered numerous reasons for investors to be wary. One overlooked, but important, data point the company included was that the average revenue it sees per subscriber has been declining and is far less than the $10 a month it charges for a standard subscription. Spotify attributes the decline to fluctuations in foreign exchange rates and discounted subscription options, which it says help it keep customers longer. There...