It is true that the IMC Rating has no validity outside the UK. However, those that do have (or have had) such a thing will have gained some valuable experience of IMC flight whilst holding this qualification.
If a pilot is in a situation in which the IMC Rating is invalid (abroad or lapsed fit this) there is a great temptation to consider experience the qualifying value rather than the bit of paper in the PPL. Some will be tempted and some won't.
You can't go any further with this question unless you wish individuals to say if they are breaking the law or not!