Who pays the messenger?
In just about every business, the last mile accounts for the bulk of the cost of the service or good being sold. Retailers get half. Insurance people get commissions. Distributors make their share.
When the internet drove the cost of some things to zero, the equation could change, because you don't need to pay a messenger when your offer is so irresistible. So, free email could be free not just because it's so cheap to run and because you have ad revenue, but because you don't need much of a marketing effort to get the word out.
As we enter a new stage of post-industrial businesses, it's easy to forget to build in the cost of the messenger. A great idea isn't a great idea unless you can pay someone to help you spread it, to help you overcome our natural inclination to ignore you or to say "no," purely out of habit.
If you're not going to plan on paying the messenger, your offering better be so remarkable and have such a viral story that your investment in product eliminates the need for media and sales.