My arguments are the following:
1) You're assessed based on your fluency, grammar, vocabulary, pronunciation and structure of ideas. So, your body language is technically not taken into account when grading your performance.
2) Your speed is recorded, so when there is any confusion in your answer examiners will re-listen to it and won't see your body language, so again - it won't count in.
3) However, we naturally sound more fluent, persuading, our speech is more coherent and pleasant to listen to when we use appropriate body language - calm, positive, confident, welcoming.
So, the conclusion for me is that it is definitely beneficial to improve gestures, facial expressions and my tone of speaking - because they will inevitably influence the whole speech. I'd like to sound confident and relaxed, not tensed, anxious and shy.