I think the main problem with the insulin level test is that it doesn't really tell you anything. Insulin level in your blood will change depending on your diet, your body requirements, etc, etc.
It MAY be indicative of something if the test was being done as a fasting test regularly - to maybe see if it changes. But the number would still change, if you, for example, started exercising more/less than before (as your insulin sensitivity changes), changed diet, lifestyle, ...
Basically, from what I understand, the level of insulin in your blood isn't REALLY indicative of anything.