*edit* Also.. any reason why people would advise against using blockchain wallet ?
It's all about trust, you are trusting someone (know their name? address? anything but nick?) to hold your bitcoins securely.
and after the linode incident if they are on a vps you are trusting their host to an extent too
if you must use these online wallets keep a minimum balance there and use a secure wallet you have control over for the bulk of your funds
The trust is that their software hasn't been compromised; but other than that you aren't trusting that they hold your bitcoins securely -- if their database was published for all to see, it's unlikely any coins could be stolen.