I have a question regarding biscuits: do they provide significant structural integrity? Or are they mainly for esthetic appeal, or when screws can't be used? I ask because I will be building a bookshelf for my niece, which will have to support a good amount of weight. I just discovered the biscuit joiner and have been practicing with it on scrap wood. I really like the way it joins wood together, but would hate for the shelf to collapse under the weight of her books. Thanks!