there's a huge difference between Darth Vader and Superman... Darth Vader wears nothing but black. Darth Vader wears a neutral color.. and on that, lets look at the characters you have named off:
John Stewart as the Green lantern...
http://www.adherents.com/lit/comics/img/g/GreenLantern_JohnStewart.jpg
black (not the color of his skin, the main color of the suit) dominates his suit
Nightwing...
http://4.bp.blogspot.com/-DHz1PT9AXuo/Tl2l2a5KdTI/AAAAAAAAAg8/_m8oNdN3FFg/s1600/nightwing.jpg
black dominates his suit. He is also a night hero.
Daredevil
http://upload.wikimedia.org/wikipedia/en/thumb/1/14/Daredevil_65.jpg/250px-Daredevil_65.jpg
the color red is the most easiest on the eye, it is also the longest wavelength in the light-spectrum... the color red is most pleasing and easy to the eye because of its wavelength... and in this case, it represents the character of "devil" in his name... no need to break up the color. You also only see this character at night, so the suit itself is mostly in darkness, with red highlights of the muscles.
Venom
http://media.comicvine.com/uploads/7/78207/2253727-sm_venom.jpg
the majority of his suit was black! his mid-section is black.
blue beetle... depends on which one you're talking about
http://www.adherents.com/lit/comics/img/b/BlueBeetle_II.jpg
again, his mid-section is BLACK.
http://2.bp.blogspot.com/_uKUJj9VMJpA/S12t2otwOmI/AAAAAAAAG3s/6zWSxqjwP9s/s400/beetle.bmp
the first blue beetle DID break up the mid-section of his suit... it was a different shade of blue, a DARKER shade of blue.
Superman Beyond
http://media.comicvine.com/uploads/5/58424/1981532-superman_beyond_00017.jpg
mid-section is black.
black hides the contours of the body, and the body of the individual... blue does not.
you also have to take into account that the suit that Darth Vader wears is NOT a body suit, but mechanical cybernetics... and the fact that Darth Vader is an evil character...
the shade of blue does not hide the contours, the intricacies, nor the anatomy of the mid section of a human being... black does.
something that all of you fans of the MOS suit might not even realize, or maybe do not remember, is what happened with the Spider-Man suit from the Sam Raimi movies... The mid-section area on the Spider-Man suit was blue, however they had a problem with the mid-section area. because it was all blue, Spider-Man's package appeared on-screen to be too big. because of this, the CG developers had to digitally reduce Maguire's mid-section to look normal... why do you think that in the new Spider-Man movie, in the behind-the-scenes photos, the suit has a black blotch area where his junk is?... it's so that it does not become so apparent on-screen, and it does not enlarge itself
i guarantee that this movie will have the same problem... and that is one of the reasons as to why breaking up the blue in the mid-section is important in a single-colored body suit. it helps in making certain parts of the body less exaggerated, whether that be the crotch area or not.
so yes, it IS important to having a break-up in the mid-section of the bodysuit... it helps distinguish the upper body from the lower body in the subconscious mind, it helps distinguish the upper body from the lower body in our peripheral vision when the superhero is going at such a fast speed he appears as a blur, and it helps reduce the unwanted exaggeration of certain parts of the body.
a full blue bodysuit will not help in post-production. they will have the same problem Sam Raimi's CG team had.