Your credibility can be questioned and it is right now, because by no stretch of the imagination is D a marginal singer. That's like saying Ornette Coleman is a marginal saxophonist b/c he does avant grade shit. The mastery one's medium is precisely what allows an artist to stray from conventions. From just about any technical standpoint (tone, timbre, pitch, phrasing), barring enunciation, D is far from marginal. It's not even about personal preference, he's from a purely technical standpoint an awesome singer who can emote the fuck out of a lyric. For you to pull Sisqo as your example tells me how disqualified you are to have this discussion.