The trunks are not essential, but the purpose they serve (breaking up the sea of blue) is fairly essential from a design perspective imo. The reason "trunk-less Superman" looks stupid is they didn't adjust the rest of Superman's suit design to compensate for the loss of color.
The only 'color' in Superman's suit is the big S, which is at the top of his suit. This is different than say Spider-Man, whose entire top half is taken up by the red spidery design, which makes the bottom of the costume free to be blue. No trunks needed. The change from red-to-blue switch at the waist accomplishes the same effect naturally.
(for my money Spider-Man's costume is by far the best designed superhero suit of all time, I could go on and on about it for paragraphs and paragraphs but I won't. It looks fantastic onscreen as well as in the Raimi movies)
Superman doesn't really have the option of going "armor-like" which makes the homogeneous color look less stupid.