Blockchain.info/wallet is a hybrid EWallet.  This can mean different things but with Blockchain.info this means that the private keys are kept locally in your browser but an encrypted copy of your wallet is stored on Blockchain.info's network (technically, also backed up on Amazon S3). 

But to use the service, they need to be operational.  There were a couple periods of extended downtime last month -- in which nearly a full day passed where access to the service was not possible.

Those with a copy of their wallet backup could easily load the file into either Multibit (or Electrum too I think) and continue to use the wallet.  But many people didn't have their Blockchain.info/wallet accounts configured so that backups get sent after each change to the wallet occurs.  So they couldn't do anything when the service was down.

The other type of EWallet is a hosted (shared) EWallet.  This is what Mt. Gox offers, as does Coinbase, Instawallet, Paytunia, bitZino, etc, .    With a service like this, you are 100% dependent on the EWallet provider remaining operational. 

Just like you don't carry all your physical cash in your wallet in back pocket (you keep some in the bank, some maybe in a safe or hidden "under the mattress"), the same goes with bitcoin. For long term storage, a cold wallet (paper wallet even, created offline) for savings, then a web and mobile app (e.g., Blockchain for Android) for spending, and maybe a hosted (shared) EWallet so that you have some coins accessible as an alternative should blockchain.info/wallet be undergoing maintenance, for example.