Puerto Rico is home to around 3.7 Million people, which more than many states, so I don't see why it should become a state. Also count in the fact that Puerto Ricans can't vote in the US Presidential Election, and since the US Constitution doesn't necessarily apply in the Territories, it's a dis-service to the people for it not to be a state. I'd also say that it could be merged with the US Virgin Islands into one state of Puerto Rico.
Right now Puerto Rico is losing their culture because the United States can put American restaurants and businesses on their soil without their consent, as they are only a territory. They also can not pay taxes, which some would say is good, however as a consequence they do not receive the benefits of taxes (ie. Police, firemen, sanitation, etc.). So Puerto Rico becoming a state would be beneficial to their people.
I personally gave OCD and the thought if having 51 states pushes me to the brink if a panic attack. Think of how many people would be driven to the brink of madness by this. So many people have OCD in America and 51 states is just horrible. The End.