From the Docs:

Why mixing interpolation and expressions is bad practice:

• It increases the complexity of the markup

• There is no guarantee that it works for every directive, because interpolation itself is a directive. If another directive accesses attribute data before interpolation has run, it will get the raw interpolation markup and not data.

• It impacts performance, as interpolation adds another watcher to the scope.

AngularJS Developer Guide – Interpolation