It also strengthens the airlines arguement that it wasn't something they were in control of, because they pushed back on time - they did their bit, but ATC wouldn't let them go.
We used to do it many years ago in the era of regular flow control issues over Spain and France as it meant you were ready to go if an earlier slot came up - we had a few flights miss a slot change because they were sat on stand and we couldn't find a tug etc...