I've heard that too, but couldn't do it either. I think it has several possible factors that propose this assertion:
1. Some say that artificial sweeteners will increase insulin response on possibly fluctuate blood sugar even though the calorie amount is minimal to none and it contains no sugar. This could possibly be seen by some as not something you would want to occur close into a show because it might hinder ultimate fat loss and tightness. Others have said that this theory is irrelevant.
2. Water is definitely the best thing to drink no matter what, but even with the added artificial sweeteners, preservatives, and color Crystal Light would be the next best thing. You also have to worry about drinking diet soda's that might be high in salt or caffeine in last week attempts at getting ready for a show. Depending upon the individual salt my need to tapered and caffeine may need to be discontinued, especially the last week before a show.
Now, besides those theories I'm not really sure, but honestly IMO I don't it really will matter too much.