We know that there are few instances in which purchasing goods and services is easier/more convenient/cheaper. In general, it is the opposite of all these things.
A merchant pays merchant processing fees. These are in the 3% range for many businesses, and in the 5% to 8% range for "high risk" merchant categories (e.g., travel, online gambling, tobacco sold via an e-commerce site, etc.)
So there are many opportunities where using bitcoins instead will cause the merchant to save money and with that being a tool to gain a competitive advantage expect these merchants to pass some of the savings on to their customers, either in the form of lower prices overall or in discounts for those paying with bitcoins.
High Risk Merchant Accounts -
https://bitcointalksearch.org/topic/high-risk-merchant-accounts-73694if one believes that bitcoins will be more desirable/valuable in the future than they are now (for whatever reason), then what rational person would spend coins instead of USD?
When I want to spend using funds that I have invested (e.g., shares of Apple) I need to sell off the specific amount of my investment in order to get the cash for my spending. I might do this to lock in the gains, or simply because I wish to use the funds for consumption rather than investment.
Wouldn't it be nice if I could pay for my purchases using the investment directly (e.g., if my landlord accepted my Apple shares as payment) rather than having to sell them at the market and withdraw the proceeds to my bank?
Bitcoin as both an investment asset as well as a trading currency allows me to spend from the money I have in an investment without having to first convert it to fiat.