Coinbase is one of the most important exchanges for cryptocurrencies due to its ease of use and creditability. It might not be the cheapest one due to its fees and I know that some of you are against centralized exchanges, but I believe that we, as a civilization, are not yet ready for fully decentralized exchanges.
For full adoption of crypto currencies, we need to attract the normal humans, who are not necessarily able or willing to deal with a decentralized exchange or even private keys.
And Coinbase is in my opinion an extremely important tool for mass adoption.