Justifying YA and Graphic Novels in the Library
In a lot of the classes I have taken there has been a huge emphasis on the mission statement. Your library's mission statement should be able to support everything you do in a library. This includes providing young adult and graphic novels. Mission statements tend to be very broad. The simple phrase "providing recreational materials for all age groups and interests" can be a simple enough way to justify including young adult and graphic novels in your collection.
I think it is very important to provide a wide range of materials to your patrons. Some people would never pick up a young adult or graphic novel unless given the chance through proper display placement. It is healthy to have a variety in the materials you provide.
What it really comes down to, though, is your community. Each community is different. Your collection should reflect your community's needs. I think that these collections are worth investing in at any library, but the amount of time and money spent investing in them will depend on your community and patrons. If you get a lot of young adults traffic or you are trying to increase young adults traffic, then you will most likely spend a lot of time and money on your young adult section. If you get a lot of requests for different graphic novels or the latest volumes in a series, you are going to want to focus time on that section. If they don't seem to be circulating much, though, maybe you only need to stick to adding the best sellers and/or award winning titles. Or maybe your young adults really stick to one genre, so you would need to focus your time on that.
I guess my bottom line is that I think it is important to provide these different genres, but to what extent is dependent upon your individual library. Knowing your patrons' preferences and having a mission statement that supports the growth of these areas will help you justify having young adult and graphic novel collections at your library should you run into any complaints or questions.