The fundamental reason why humans have developed currency is because no other system is equipped to properly cope with the complexities of goods and services. Bartering already was not feasible thousands of years ago on a large scale, and it's even less realistic a model today.
Let's say you want to buy a car. How do you barter for that? Drive up with a truck load of corn? Or do you take a job at Ford and build your own car? Are you also going to stamp your own steel, mix your own paint, manufacturer your own fastners? What about all the electronics going into that car?
Perhaps someday we'll have some incredible utopia were everyone selflessly works for free and robots do most work for us. But until that day comes we need money.
I'd argue Bitcoin could be as legitimate a currency as anything else, but the problem is that it's pegged to nothing. It's value is entirely defined by being a get-rich-quick scheme. Sure, there are plenty of people playing similar games with legitimate currencies, but they're not the ones defining its value because those currencies are still tied to more tangible things.