There is no compelling reason to use an EIN to open a "business" credit card as a sole proprietor. The card is issued to an individual, not to a company. You must still provide your Social Security Number on each application, you are still personally responsible for the debt. Creating additional EIN's does not make you more creditworthy or allow you to open more cards.

The IRS has no special interest in "business" credit cards. If you have a real business, and if you commingle personal and business expenses, this can create tax problems, but this is due to your bookkeeping procedures, not to the type of credit card. "Business credit card" is a marketing term. It's how a card is used that matters for tax purposes.

Simply opening a "business" credit card does not create a business for tax purposes.