I do not agree that an experienced IR pilot goes rusty if not used for six months and becomes a danger.
I agree.
However, as with any skill it will degrade. When I skied for a season I was much better at the end than I am now. Flying a modern aircraft with good avionics in IMC is not hard, flying the same aircraft partial panel in cr*p weather on one engine is hard. If you are current you should make a good job of it, if you arent current, you will struggle.
The IMC rating involves around half the training of an IR. The tolerances are not as high as an IR. For anyone who has done both it should be quite clear the additional elements involved with an IR that produce a "safer" pilot - the trouble is you will find that most people who hold a view on this subject have not done both and therefore sadly - dont know what they are talking about.