The only ass-shot in ME 2 i remember was when you/Shepard had his little talk with Miranda in which she boasted about her design being perfect. She even mentions her looks and that is about the time this ass-shot takes places. So, IMO, at an appropriate time and not just for 'teh horny'.
https://www.youtube.com/watch?v=G2mmzgeCxBw
Why that scene needs a rework eludes me. The camera-work and the model-placement on the other hand need a rework. Too often it seems that the characters look past each other when they talk.