I think we will have to agree to disagree on this one ;)
Tone of voice is certainly helpful. It can create a calming soothing environment, or an excitable atmosphere or a threatening feel. But it is NOT a reinforcer in the scientific sense of the word.
I always maintain that treats are not everything and have said multiple times that scratches, comfort and play are all primary reinforcers that will encourage repeat behaviours. But it is undeniable that food is a very strong reinforcer and as such you can get great results by using it.
I personally train all behaviours without pressure release successfully but I know most people don't want to do that. However tricks like bowing are a perfect opportunity to just have some fun and experiment with other methods and approaches that you wouldn't normally be using. Who knows, maybe then you could experiment with other more 'work-like' behaviours...