Not to mention the chances of airframe icing...
As someone said earlier, yes, you can get airframe icing in a few well-understood conditions at +15C, but if the temperature drops below 0C, the chance all of a sudden become much greater.
It's also handy, if you keep an eye on the OAT during the ascent/descent, to find inversions and a few other weather fenomena that are highly dependent on temperature gradients.
And lastly, most aircraft performance tables, such as takeoff and landing distances, are temperature dependent.
So I wouldn't want to do away with the OAT just yet.