This brings risk when a user accustomed to a particular decimal point location switches to another client. A user in the habit of typing "100" for 0.1 BTC may accidentally overspend by a factor of 1000.
So we should advocate software that provides customizable transaction/fee size limits and warnings, regardless of whether there is a choice of btc/mBTC/uBTC preference.
We need warnings such as: "You are asking to send 270 BITCOINS but you have requested alerts for payments over 2 BTC. Previous payments to this payee have not exceeded .5 BTC (500 mBTC)."
And: "You have not included any decimal point in the amount of this transaction. Do you really intend to send 527 BITCOINS?"
And enforced limits such as: "You have specified a 0.5 BITCOIN FEE for this transaction. Your maximum fee setting of 0.5 millibits currently prohibits this."
(To limit such risks, one should of course keep higher balances separate in cold wallets.)
It would be useful, at least for the foreseeable future, to include a fiat amount akin to bitcoin wallet on android. I can enter the amount I want to spend in either and it fills out the approx. amount in the other field. If someone is buying a bar of soap and fills in 0.5btc and see's the gbp amount of £281 they are going to think twice before hitting send.