What is the purpose of supplements? To provide a gamer with something they do not have time or desire to create on their own. Admit it, after a core rule set, a gamer does not ever need another book again (and some would even say that a person could create their own game system if they wanted to). If you want to add Blood Mages to your game, you can create it on your own. If you want a city for the the players to explore, you can create it on your own. But there are many times that myself and others do not want to create something on our own; that is when we buy supplements and that is why designers and companies can stay in business making us supplements.
But not all supplements are created equal. What do I want out of a supplement? To provide me with something I do not want to create - and that is the key thing - something I do not want to create. This is where many supplements fail at their purpose. For example, if I buy a supplement describing a city, I want a city I can use without requiring me to create anything for it. All too often I will get such a city book and they will give descriptions of the various buildings but then leave some without descriptions. They do this "so the reader can add in their own ideas". If I wanted to add in my own ideas I wouldn't have bought the supplement in the first place, I would have just written it myself. And if I did have something I wanted to add, I would simply replace one of the supplements descriptions with my own. To me "so the reader can add in their own ideas" is the same as the designer saying "I ran out of ideas so I'm leaving this blank".
So, if you are working on a supplement or are thinking of writing one for public consumption, remember what the goal is for such material...to do what I don't what to do.