"Why should Puerto Rico become a U.S. state?"
Because the Constitution of the United States does not provide for an internal apartheid system, where four million Americans live at the whim of Congress. I say again, the residents of Puerto Rico are Americans already.
"Why should't they become a country on their own rather then a quasi part of the U.S.?"
The bonds between Puerto Rico and the rest of the States are too tight to break. There are more Puerto Ricans living on the mainland than on the island. These are not just the stereotypical ghetto Puerto Ricans of certain northeastern US enclaves. These are Americans that live and work everywhere. Likewise, the island is home to many transplants from the mainland. And so on (it would take me time I don't have to give you all the reasons, but just one reason should suffice: we are Americans, already).
"Doesn't the U.S. spend billions each year on Puerto Rico?"
Doesn't the U.S. spend billions of dollars everywhere? These "gifts" to Puerto Rico are another myth. If I can cede to you one thing on this matter is that Statehood will put Puerto Rico in equal footing tax wise as the rest of the states. I know that a State of Puerto Rico would be strongly a Federalist state, and we would fight DC's intromissions to the benefit of the rest of the states.
I don't underestimate the sacrifices of the people of Puerto Rico to be citizens. I served in the military with many and they served wonderfully.
I am not saying that Puerto Ricans are not worthy. I just think the differences of language, culture and lifestyles are very different then the vast majoprity of Americans. Also the economic differences are vast. The idea of graft and corru[tion within police and populace are too different.
To bring the infrastructure to U.S. standards as well as the level life would be too great.
And to be honest, for what?
What advantage is there for the U.S.?