A couple of my thoughts..
I bet in the DVD commentaries it will be said that the solid black suit with no texture would have or did look bad on screen. I am willing to bet that there are screentests of the all black suit, and it didn't work. Off the top of my head, I can think of these reasons why: A) It would be hard to show any detail of the suit. In low-light condidtions, Spidey would probably just look like a black blob, and from the trailer, it looks like black Spidey does most of his work at night. B) the suit is more grey than black for the exact same reasons stated above. At least grey throws a little light back at the camera, and that light will show details that would not be seen in a solid black, non-textured suit. C) Most of the audience going to the movie are not hard-core Spidey fans. The fact that they went with BASICALLY a grey version of the red and blue is to facilitate transition for the casual fan. Also, remember that the new trailer shows that CGI modifications were made to the basic suit. For example, in the suit ripping sequence, the webbing on the suit is noticably broader-spaced than the traditional suit. I am sure there are more differences that will be brought out in subsequent trailers and the movie.
As an aside, since we have already seen that there has been mods to the suit from the first trailer to the last, I am ALMOST willing to bet that all of the black Spidey pics/videos we have seen, even the newest trailer, are all PRE RENDERED. Meaning we could get a suit in the movie that is different still from the update we saw in the last trailer. They could be sitting on the post rendered suit as a surprise. I know not much of a chance, but there is the possiblity. One reason I say this is in a lot of the shots, it looks like there is a layer of CGI over Spidey when you know he is actually there. I speak specifically of the scene where Spidey files off part of Sandman's head with the subway. Look at Spidey's arm holding Sandman's head. It just doesn't look rendered the same way as Sandman. It looks oddly out of focus with the rest of the scene, even compared to Sandman.
Anyhow, I'm not trying to start a conspiracy here, just saying we may not have seen the final render of the black suit at all.