When a movie is completed, it is reviewed by the MPAA (Motion Picture Association of America). After it is reviewed, it is released in theaters and/or DVD with the rating attached.
When a movie is released on DVD after being in theaters, they sometimes add things to the movie and it hasn't been reviewed by the MPAA. So, they have no official rating to give it. As a result, it's UNRATED.
It doesn't necessarily mean it's worse than 'R'. It could be, or it is simply the same.
I would. I prefer "special edition", "unrated", "director's cut", etc... over regular versions.
Some people don't like unrated versions because they think it's worse than the movie.
My parents would hesitate when buying a DVD that's unrated because they don't know what's on it. When they hear "unrated", they think of movies like There's Something About Mary and Me, Myself, & Irene and American Pie... just hardcore and disgusting.