Fair point. But I would point out that the only things this works with are things we have thoroughly mastered (e.g. walking). People have issues talking and driving (e.g. in parts of Europe it's illegal to talk on a cell while driving because of all the accidents it causes).

In my experience it depends on the amount of processing required. If I'm driving a straight away with traffic going mostly the same speed I can hold complex conversations. As soon as something happens on the road I drop the conversation until my attention isn't needed on the road anymore.