First and foremost, the campaign is NOT to automatically cast Glover as Spidey. It's simply to grant an actor who's passionate about the role an opportunity to audition. He'd still have to earn it just like everyone else. It's not affirmative action or anything like that.
Another comment that's come up is that Spidey has been portrayed as white for forty years; if you cast a black man, it'd fly in the face of all that continuity. News flash- only comic fans give a shit about continuity. As recent comic stories have shown, the editors don't seem to care about continuity, and you can be sure that Hollywood doesn't care about continuity.
And then there's the comments that run the line of, "If you want a black superhero, then make one!" There are many black characters, but they never get the kind of support from publishers and other media adaptors that white heroes get. When they do allow them to star, they're often stereotyped. Hell, it's a miracle that they didn't have Wesley Snipes talk in jive for Blade, though admittedly, his dialogue was often silly enough as it was.
Then there's the most stupid comment I've seen: "If you cast Spider-Man as black, why not have a white guy play Frederick Douglass or Malcolm X?"
That's a level of stupid so mind-boggling I have trouble trying to answer, but here goes. Malcolm X and Frederick Douglass were historical figures, and their blackness is supported by numerous primary and secondary sources. Spider-Man, by contrast, is a GODDAMNED FICTIONAL CHARACTER!!! It's like that scene in Jay and Silent Bob Strike Back.
What the naysayers' argument basically boils down to is that there's a tradition of portraying Spider-Man as a white guy. Traditions are not ironclad, and I see no good reason why this tradition should remain so. Change is a part of comics, which is why Superman's not single anymore.
Captain Marvel is still available, though.
And then consider the alternatives. Of the five actors that are allegedly in the front-running, two of them are British. A third one, while ostensibly American, has had most of his professional acting jobs in the United Kingdom, so he may as well be British. No doubt he's defected to the Crown. That means there's a 3 in 5 chance that we'll get a Spider-Man who says things like "Pip, pip! Fancy a scone, Doctor Octopus? Let's make it fast, so I can pop back over to my flat and snog Mary Jane!" That makes me so sick to my stomach that I'm liable to throw up my very American breakfast, which, for the record, consisted of Belgian waffles, French toast, and Canadian Bacon.
A vote for Donald Glover is not just a vote for a black actor, but a vote for the old Red, White, and Blue. It doesn't matter if Spidey is played by an African-American, so long as Spidey is American.
Old Glory and Spider-Man go together like cheese and bacon!