There are powerful inducements to holding an Amex credit card.American Express (AXP 1.76%) is one of the most recognized and familiar payment card brands not only in its namesake country, but throughout the world. The specialty financial company has a massive customer base and runs a business that is consistently profitable and continually growing. That's not the foundational reason its stock is a good investment, however. There is one element of the company that makes it unique, and that uniqueness will re ...