The circumstances where it is not the case are either you have many spare parts in your garage already or you have very specific parts that you want to have on your bike, where you are going to spend money on those parts even if you buy a complete. It is essentially never the case that you will buy a frame and all the parts (and possibly tools to put the bike together as well) and spend less than you would if you were to just buy the complete bike. Attempting to do so will mean that you will end up purchasing parts that are worse than you would if you bought the complete bike. It is fun to build up a bike but if you choose to build up a bike you are going to actually pay extra to do so. Worth it? I think most people who do build up a bike feel like it was worth it but many people also feel like they wish they had saved money and time. It depends on your personality.