Coinbase makes Bitcoin easy for the common man. No need to set up a wallet, no need to manage keys etc. Coinbase also pays mining fees for you when you spend. You can send money from Coinbase wallet to Coinbase wallet instantly without needing to wait an hour for 6 confirms, etc. They also provide a merchant service for stores to accept bitcoin, they convert it to USD and depost it to the merchants bank.

So, there are advantages.

Yes, you have to trust them to not go away. I don't think they are going anywhere.

It's totally up to you, I'm just answering your questions.