That always seemed hypocritical to me. Roger came to the band with this grand concept and bunch of it already written (in his head anyway, but some also literally written out), pretty much calls all the shots during the rehearsals and recording sessions, then complains that the others aren't contributing as much as he is.
As for writing credits, I've seen many quotes from many artists saying that you just can't believe them. They're political, financial, economical, whatever you want to call it, but never an accurate reflection of who "wrote" the song. Because everyone has different ideas about what the means exactly. When I listen to Animals, my favorite Pink Floyd album, I hear Rick Wright's keyboards and think they're brilliant, and I love the way he does things which are exactly perfect for the song, no more and no less. He came up with those parts. Maybe Roger or David or whoever "wrote" the song gave him some guidance as to what they had in mind there, but Rick played them, and in that sense "wrote" those parts. So if anyone's arguments against Rick's contributions to Pink Floyd are based on the writing credits, yet they claim to have actually listened to the albums, then I can't take them seriously.