SO and I take turns pulling each other on our commute (well, I do most of the pulling, it seems.) We talk about the day, road conditions etc. at 35 km/h quite easily. If you're drafting, the person in front has to be calling out hazards - you are following too closely to brake or avoid anything. If you're far enough back that you can fend for yourself, you're not in the sweet spot. You know pretty quick if you're drafting someone - you'll be pedalling a lot less than the guy in front of you.

I consider it rude to draft others without asking or otherwise indicating your presence - when it's raining, do you go up and stand under someone else's umbrella, stepping on their heels? Thankfully I've never been wheelsucked yet - we go too fast for the average commuter around here, and too slow for the roadies to bother.