I wouldn't go so far as to say we have a solemn duty to always take such issues into account. But as a designer/developer, it is certainly something I myself would spare a thought for because - putting on my consumer hat - I am directly affected by the problem of shitty data plans at present and I'd like to think it's something that designers/developers would be aware of, regardless of whether they then make an informed choice one way or the other.
> Do we have a responsibility to do the opposite, as an industry exerting pressure on carriers to increase cap levels?
I'd like to think this would work. But I really just can't see it.
I too would be leaning towards designing for ther reality. But it seems we're in the time of 'case by case basis' on this issue. I don't think there's a single correct answer.