IO540 i totally hear where you're coming from.
When i trained our CFI was more than happy to go flying if our AA5 was out of limits. Taking into account his years of experience I had no doubt that he could get it off the dirt and into a climb without killing anyone but i didn't really think it was a good example to set to a student (which i was at the time).
Although i agree being slightly out of W&B limits may not necessarily mean you can't get the kite into the air, IF something does happen, the PIC is fully responsible and before a flight he or she would have to ask themselves, is flying just this once out of weight+balance limits worth my licence?