If you can't export your private key in a click of a button then Its useless I'd say also a lot of people are using the ERC20 tokens and not ethereum itself so again, they will be losing a lot of users If they are not supporting that, I say Myetherwallet is still a better alternative for the moment.
this is the big issue i see. at first, i was excited, since MEW is occasionally unusable/service down.
with the ICO craze still in full swing, and airdrops still flowing left and right, it'd be great to have more options for reliable web wallets with access to ERC tokens. i don't really need another web wallet just to store ETH. i have a desktop client for that. hopefully they will add support for ERC tokens in the next few months. i'm sure there is demand for it from their customers.
Well, MyEtherWallet is just a client so you can see and manage your wallet from your key. Since you have access to your private key, you can access your wallet anywhere, even if the wallet is down. That's why MEW is the best option if you want to create a ETH wallet.
Blockchain is essentially hosting your private keys for you. Not the best option and not offering all the freedom you should have.