First, I agree with your understanding of "on your left" and the d-bag that didn't want to change lanes can suck it....
In many (most?) places, riding two abreast on the road is legal, as long as you aren't impeding traffic. Ettiquette-wise, I think it's fine on the road, in the lane, assuming you're acting in accordance with the law. However, I'd say to avoid doing it on trails and in bike lanes, where there is generally less room to pass. I certainly wouldn't call someone out for doing it, but I think that it's a bit inconsiderate and shows a lack of situational awareness. If you're out on a long stretch of trail with few people around, by all means...just keep an eye out behind you so you can move over if faster riders need to pass. I'm not quite sure what the "dicks" comment was thrown out for...in all but the most egregious misbehaviors I just keep my mouth shut on the trail....people need to keep their road rage off the trail.