Browsing through this thread, the main argument seems to be that they are most comfortable with the Mac APIs and think it would be harder to rewrite their stack using different technology. They would have to re-apply the new stack onto millions of existing images and make sure the results 100% match the previous version, which is a problem that you don't have launching a new API.
So I'm not quite sure that I understand this reasoning in your case, as the operations performed (scaling, cropping, watermarking, flipping, filtering) are available in just about any image processing pipeline, and not really linked to anything that Quartz or Mac does particularly well.
So I'm not quite sure that I understand this reasoning in your case, as the operations performed (scaling, cropping, watermarking, flipping, filtering) are available in just about any image processing pipeline, and not really linked to anything that Quartz or Mac does particularly well.