Cosmo, while being the source of Singin' in the Rain's key ideas, also has a certain danger or risk associated with his character. This is most simply illustrated by his failure to be romantically partnered within the film.
That Cosmo is sexually ambitious is established at several points in the film; he utters the infamous cliché of "sure, I can get you into the movies" at "The Royal Rascals" premiere party, the dance with the headless mannequin during the "Make 'em Laugh" sequence ends with a kiss and slap in the face, and he explicitly desires a kiss from Kathy at the end of the "Good Morning" sequence.
While the film's refusal to accommodate Cosmo's desires (it would have been relatively easy to provide a whacky and zany side kick to Kathy who could have been Cosmo's romantic interest) could be considered tangential to the film, it does suggest something about the 'uncontrolled' nature of the character in his role as the mad fool. There is a risk associated with his creativity, a risk that the film refuses to characterise as negative but more simply as a wildness that leaves him 'outside' the terms of the romance that the film relies upon. That the romance is a central term in the film's argument is crucial here, for the couple's role (and their place within the more abstract argument that I've characterised as 'musical cinema') is to mediate and facilitate the conflicting demands of 'creativity' and the pragmatic world.
This also provides Cosmo with the permission to mock authority (such as the "Moses supposes" sequence and his sarcasm towards Lina), but it accounts for why Don regards Cosmo's opinion as significant.
Created in 1998 by Adrian Miles, details, republished 2006.