Well, if ATC clears You to maintain a heading, is not this what they want and expect You to comply with, with all its consequences? If they donīt like the drift the wind imposes on one of the aircraft under their control, they can always amend the heading "for drift correction", as some units tend to explain when giving such instructions.
If they had wanted runway track, they could have given such a clearance instead. I guess that they donīt do this because they are aware that not all aircraft are able to accept an ad hoc "track" clearance. Consider a 320 (able to fly a cleared track) and a Dash 8 (unable for a track) departing parallel runways, both cleared for runway heading, and the 320 crew decides to use track instead. All it takes is a matching crosswind for the tracks to converge. With proper headings, both would be subject to roughly the same drift.