I can exchange them fast and pay cash for my things
Well i think All of users can exchange bitcoins with fast, but not all of users bitcoins can pay things using bitcoins on their real life. Because there are not many stores accept bitcoins for payment gateway. It is true problem usage bitcoins today.
Of course you need to exchange first into your fiat before you use your money in bitcoin. That's the most people doing, even I usually do that, one or two week . I use my money to complete my weeks bill and buy my college stuff.