I am confused. If you leave the bees adequate stores of their own honey - why do we need to leave a sugar board?
Or as Finman will tell you it is a matter of economics. Honey is more valuable than sugar, so rob the more expensive honey and feed them the cheaper sugar.
I tend to have the same philosophy as golfpsycho, sugar is cheap insurance that your bees don't starve. If you make a sugar board, there is no waste. What they don't consume this year, you can save and give back to them next year.