You've decided to bake a batch of cookies, but as you are preparing the ingredients you realize you are out of butter or margarine sticks...you don't want to make a trip to the store, so you'll just use the butter in the tub (i.e. spreadable butter). It'll be fine, right?
Probably not. In fact, you'd be better served in nearly every instance to either go to the store or wait to bake up those cookies until you have sticks of butter or margarine. Why? Most spreads, light margarines, or whipped margarines contain more liquid than regular stick butter and margarine. The result - cookies that won't brown, are either soggy or overly tough, and in some cases, even oily.
If you choose to use margarine, rather than real butter, be sure that it contains 80% fat. You can typically interchange margarine sticks for real butter sticks, but in the cases of candy making or when making sugar cookies, shortbreads, or pastry crusts, always use the real stuff.