its about convenience.
not having to store dozen gigs of blockchain on your ssd. or keeping it in sync over a tiny limited data plan
Well if your willing to take that risk for convenience thats up to you, unfortunately securing your wealth isn't a matter of convenience. Its more convenient for me to store my cash under my bed but its far riskier than storing it in a bank even though its less convenient.
If the block chain is a problem use an SPV wallet. 18gb is nothing now days.
I control the private keys of my blockchain.info hot wallet and i don't have a big part of my bitcoins in this wallet. But i am not sure it is the case of 1.5 million users. I am sure a lot of them are not able to get their bitcoins if the site is down.
But thats their problem. What I said still stands, if your going to trust your coins to a third party know the risk you are taking and if you don't control the private keys yourself then your doing it wrong. The whole premise of Bitcoin is not trusting third parties. If you trust a third party whatever happens is your fault. It sounds harsh but learn to use what we've been given correctly and there are no problems. I mean its not like the site went black for days, it was an hour or so and people are freaking out. If they are managing their risk correctly they would not be freaking out. Unfortunately people are going to continue to get burnt until they learn.