So anybody in a film whose name is not in the title is superfluous?
That doesn't tell the audience anything about him, his powerset, etc. (And he's not alone in this. Guy Gardner is probably equally mystifying for the people who don't spot the Green Lantern logo and know what it is.)
For most movies, characters are differentiated by dialog and their actions, and all start from the same general baseline. Like, for most films, characters are normal humans and we learn about their goals and motivations over the course of the movie. Morrus offered Harry Potter as films with many characters. Characters in the HP universe have an important additional dimension to them -- are they magical or a muggle? That takes extra dialog or other indicators to convey to the audience, which takes up time. Most witches and wizards in the HP universe have similar abilities (albeit at different levels of skill and power). When someone is significantly different in what they can do (like Professor Lupin) it is called out, which again takes time. But most of the other characters are other magical students, with similar abilities to Harry (short of his special background).
Superhero films have the same thing, but it is amplified because supers can have wildly different powers than Harry's classmates. So films have to communicate the character's motivations and personality, the fact that they are a super (costumes provide a visual shortcut for this), and then what their powers are. So the more supers involved, the more time is required by the film to relay this information. This can be done efficiently, but it is hard to do well. It can help to have superheroes that most of the general public is familiar with, like Superman, Batman, Spider-Man, etc. As Whizbang Dustyboots points out, the additional supers here are unfamiliar to most people outside of comic readers, so I understand the concern some folks have.
All that being said, I do have high hopes for this movie. James Gunn has shown his ability to manage obscure characters well without sacrificing story or character development. I know when I first heard that Marvel was making a Guardians of the Galaxy movie I was concerned that an entire group of obscure characters would be too difficult to adapt, and some characters would get short-changed. I was pleased to be proved wrong.