You do understand the peer review process don't you?
Here is how one physicist describes him:
Michio Kaku made a fundamental and significant advance in physics, he created light-cone string field theory, following Mandelstam's light-cone formulation of string theory, along with Kikkawa. This contribution was central, because it was the first definition of string theory which was Hamiltonian, meaning it could tell you a detailed story of how strings split and join in space time. It also allowed you to produce a detailed description of the Hilbert space of string theory which is not a scattering space.
The thing about physics is that it has become annoyingly politicized, with two branches--- the technical branch which produces all the results, and the popularization branch which gets all the political clout. This division is extremely unfortunate, but it is a byproduct of the fact that nobody in the general public reads the technical literature. So people with immense technical clout, like Georgio Parisi, are incomparably less politically relevant than those with popular books, like Brian Greene.
This is a plea to the general public: please read the technical literature. I mean it. Please read it. It is a precious production of our culture, it is the main thing we will be leaving to future generations. The 20th century physics literature is our Shakespeare, it is our Homer, it is the thing that defines our cultural legacy to the largest extent. It is not acceptable to have this literature be the domain of an elite, it must be universally appreciated.
Under these circumstances, there will be no need for Michio Kaku to go around selling himself to mass media, he would have been appreciated for his technical contributions, without any need for him to become a publicity hound. But since we don't live in such a world, he has become a publicity hound. I think it is a bit of a shame, but it will never take away his earlier achievements. String field theory has receded somewhat from the main focus, now that we have AdS/CFT, since string field theory is not the most fundamental way to view string theory. But it is a valid technique, and it has led to many insights, and it is still the most economical formuation of string theory, and it is still something to celebrate. Michio Kaku might not have written Shakespeare's works all by himself, but he wrote "Macbeth", and I don't know how anyone can speak ill of a person who made such a contribution.
To explain why the other answer here is wrong: while Kaku's contribution to string theory was not as enormous as Mandelstam's, Venziano's, Schwartz's or Scherk's, he did do something important in an important field, at a time when nobody took the field seriously. The people in the field were hounded and rejected, and it is impossible for him to have become a leader of physics based on his string field theory work, because people laughed at string theory in the 1970s.
The "leaders" of the field, Schwarz, Green, these folks were isolated in small departments and had no influence. The other "leaders", like Yoneya, and I'm talking about a whole generation of physicists, were just purged from the field. They were heckled for being crazy, and for denying quarks. Scherk took it to heart, and actually went crazy. It's a terrible story. You can't fault Kaku for being political in late life. While it would be nice if Mandelstam got the public recognition, Mandelstam is a very old man now. Somebody has to be a face for early string theory, and Kaku is as good a choice as any, he made a great contribution.
In terms of technical achievement, string field theory is still useful and relevant, although it is less central than AdS/CFT and holography. So what. It's important.
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