Regardless of whether you think Puerto Rico should become the 51st state in the United States of America, saying it is not sufficiently "American" is problematic. There is nothing sufficiently American - while people say baseball or apple pie are symbolic of the US, I hate watching or playing baseball, and cannot stand the taste of apple pie - and yet, I am still proudly American. Americanism extends beyond trivial things like language and skin color, and there is no reason why Puerto Rico is any less American than California or New Hampshire besides the fact that it's not a state in the USA.
Puerto Rico gets all the good parts of the United States of America but none of the bad. They do not have to pay US taxes but get our defense. They should become a state and join the United States of America. As a matter of fact so should Guam.