One thing that Bitcoin detractors like to bring up is comparing BTC against USD with the 'you have to pay your taxes in USD' argument. I have no expectation that BTC replaces USD anytime within the next 20 or more years. However, it doesn't have to - BTC has value because it's more useful than the USD in a number of ways.
BTC is primarily used in manners that push the envelope of allowed activity. This includes illegal activity such as purchasing drugs, credit card data or other such flat out illegal transactions. However, it also includes legal transactions that are merely frowned upon by certain elements of society. As an example, BTC is more useful than the USD/Credit Cards/PayPal in most targeted industries listed here:
Investors may not like it, but this is what gives BTC most of the value it has today.