If you can't get a single credit card, then your credit score is pretty bad. Every time you keep trying to apply for a card and get denied, it's hitting your credit score even more.
Just because a certain company denied me for a credit card, that doesn't mean that my credit score is "pretty bad." You can't just assume that a certain company's decisions is representative of the entire pool of credit card companies. That's absurd.
And for your situation, adding a credit card isn't going to fix what you're hoping for it to do. It's merely a BACKUP source of funds in case coinbase tries to debit your bank account and it has insufficient funds, it'll get charged to your credit card instead. You still have to wait a week to get your coins since funds come from your bank account, and there's no way to make the credit card as your primary source of payment.
Coinbase specifically says that if you add a credit card, the 5-day delay is removed and your purchases of bitcoins become instantaneous. Don't think that what I'm conveying in my first post is based on my own assumptions...
