Artist should be motivated by anything and everything. Should they feel they have to motivated by the needs of society? No, of course not. That does not mean you should not be able, or allowed to be motivated by the needs of society. Where ever they can get that inspiration from is what should motivate them.
Sure, I believe artists should be motivated by the needs of society. Society has a lot of needs so there's a lot to choose from when it comes to that. But, even with that, artists have the power to move people and if their art inspires change, then this could be a very beneficial thing.
Art is an expression of human emotion, experience, view, and self. While it can be, it should not be required to act as a gift or influence on society. Artists should be motivated by what inspires them, not by what is expected of them. Art is not a public service, it is a personal love.
Art should be based on the feeling of the artist. Art should be defined by the person who created it and if it is not liked by society, then society can create their own art that makes them feel happy. If the creator of the art feels happy then society should feel happy for that person.
Artists should feel they must be motivated by one specific thing for a certain reason. Artists need to feel free and be able to just let those creative juices flow. At times, if the needs of society is what motivates them, than yes they should use that. It won't always be where they get their motivation though, and should not have to be.
I'm sorry but an artist should create what he or she thinks is art. Sometimes these things may coincide with the needs of society but it is not a must. The definition of art demands that it not be subjected to the pressures of what society needs or deems to be acceptable.