It's good idea because on demo account even fail only virtual money, but some trader might they dare to take these risk on news time, because sometime impact news giving larger movement than usual, but unfortanetely effect news is uncertainty
If there was no slippage then we could have traded the news. The stop loss functions does not work on that fast paced market. And if a trade goes wrong then losses become bigger at those time because stop loss does not hit where it is supposed to be.