I would expect any pilot with a modicum of common sense, who has been told to turn left/right x degrees to maintain that heading.
How could anyone mistake that instruction for being on their own nav??
When would I use it? If i see two aircraft in conflict and want to do a climb through or whatever, I use my skills (ok, and vector lines) to project tracks, then 'tweak' the observed track on the subject aircraft. I could not give a monkeys what the resultant heading is, as long as it takes the aircraft in the direction I want it.
This is why the turn left/right x degrees usually involves a small(ish) change. If I did not know the heading but wanted to turn the aircraft a long way, for whatever reason, I would grab a decent handful then say something like 'turn left heading 345', if it is glaringly obvious that not matter what the wind is, the aircraft is nowhere near that heading at the time.
I do not have time to say to every aircraft either "XYZ report your heading" ,await reply, "XYZ roger, turn right heading" or "XYZ turn right 20 degrees continue as a radar heading and report it"!!!
I am talking mainly from a CAS perspective, where instructions are mandatory, but even outcas, if talking to a professional pilot, I would expect the same thoughts.
Maybe not ideal phraseology, but common sense if you take a second to think about it.
