Far too often the relationship between ad agency, client and researcher is difficult to the detriment of the brand.
It is not about testing the creative, but rather making a judgement about how people are likely to react both emotionally and rationally to an idea. From this we can make a judgement on the likely impact upon behaviour.
There is not one size fits all yardstick of creative. Different objectives as well as different style/ types of creative all need to be judged on their own merit