This is another old chestnut. And I still haven't heard what seems to me to be an adequate explanation. The difference is not between 'radar heading' and 'own nav'. It's between 'radar heading' and just plain 'heading'. If a controller tells you when you're under radar control in controlled airspace, 'turn left heading 240 degrees', you don't interpret that to mean "ah but he didn't say RADAR heading so he won't mind if I wander around a bit between 235 and 245".
And if the heading's been assigned for the guys in the next sector, surely the controller just says "continue on the heading and contact Next Guy on xxx.yy"?
NS
