When I first started my career in sales and marketing in the 80's the one tool you absolutely needed was an American Express Card.
Often I was dispatched to open up a new geography for my company. In those situtations having the card was great, I could buy air tickets, book hotel rooms, hire translators, rent cars, entertain (I mean negotiate) with customers and local partners. I could get things rolling without having to see my banker for an increase in my credit limit before each trip or project.
Today with video conferencing, the Internet, netbooks and devices I don't know if the card is as important. There's much less travel and entertainment and more online connecting for rapid business development.
Today, the one thing I absolutely need to do business is....my Blackberry! But hey, I still pack my AMEX card...in case.