Ultimately, for the purposes of Thor: Ragnarok, The Hulk is a buddy/sidekick to the titular Odinson. So if he doesn't have a level of speech that's up to snuff to a certain level, hearing him have extended conversations is going to be tough on the audience's ears, as well as the voice of The Hulk. Lou Ferrigno also implied during his interview with ComicBook.com that he might be doing something special in the Marvel Cinematic Universe in the future, though he wouldn't elaborate on what this will entail. So maybe these opinions are held for some important, yet unknown, reasons that we'll find out about in Phase 4.