Why so many pin-ups? Simply because pin-ups are a really good source of character development. And there is a lot that you can learn about composition simply by placing a person in an environment and getting lighting right, the expression right, getting coloration right, and getting the context right. It can be really good practice for more complex scenes. On the flip side, it can be frustrating to be bombarded with pin-ups that seem so formula... big-busted, white Vicki's who are scantilly clad. On the other hand, there are a few artists out there who have tried to break the mold. (So says the guy who put a penis on one of his female pin-ups...) This is not the first time this issue has been brought up. Frankly, the answer has always been that if it is not something that appeals to you then don't look at it. However, more constructively, if you feel that there is some other idea that you feel people will gravitate towards then go with it :)