A mobile device could potentially be stolen/lost, even if the point of the theft of the device is not to steal the bitcoin they would be significant risk.
Online wallets carry their own risk that is separate from using MultiBit in this manner. More risk? I am not sure
Everything in life has a risk. Even if someone secures their bitcoins in the most effective way, they can't rule out that they might have an accident tomorrow, leaving the coins unspendable if only they know about how to retrieve them (passwords, cold storage location, etc).
The problem with online wallets is when they actually know your private keys. They could one day just decide to spend them without your consent. As far as I know, Blockchain.info encrypts the keys so that they are only known in the browser at the time the user logs in, and never by Blockchain.info themselves.