To expand on my early point about will the PB sell as opposed to is the idol "teh smex", the girl who put out the most PB's was Kamei Eri. Was she the best looking girl in the history of MM/H!P? Sorry, Eri-wota's but she was barely third in her MM generation (behind Fujimoto and Michishige) and wasn't as good of a model as the fourth (Reina). But, for whatever reason, her photobooks sold so they made as many of them as they could.
I think that nearly every member of MM should get the chance to do at least two photobooks, because you never know how they are going to sell. Some member who no one pays attention to might find their niche as a PB girl.
Now, as for the question about whether or not some member should gain or lose weight before getting or doing a PB, I think it is really common for girls who do get a PB to diet/firm up/get a tan on their own because the want to look good. I don't think they need to be told to do so.