Unless you live in a US/Europe-centric bubble, the average Joe doesn't even have a credit/debit card. The average Joe however does have a cell phone, and many people even have smart phones. Other phone based payment systems in developing economies charge fees near 20%. Bitcoin will blow them out of the water, while also opening up new markets to existing online merchants. It will also reduce the barriers to entry for new online merchants that wish to serve these markets.
Many developing countries have relatively cheap domestic payment systems like Mpesa in East Africa or Alipay in China. I don't see bitcoin forming a threat to those systems. Fees are nowhere near 20% and you don't have to worry about the insane bitcoin volatility.