I respect you position, and if all mills (and all variations of output from them) were certified, traceability would be less of an issue. Certifications of mills is certainly important, but traceability of the product is where chain-of-custody certification comes in. Individual mills and converting facilities owned by a larger entity may or may not be certified, and therefore many certified paper companies offer both certified and non-certified products. As printers, we are not just consumers, but suppliers for our clients (and theirs) and therefore should take some responsibility for material traceability (should we so choose). Material traceability is hardly new to other industries and plays a key role in ensuring product quality and accountability (metal alloys, food industry, etc). For printing, end product quality is not really the issue, but for ensuring the components used in manufacturing are aligned with the principles, criteria, and standards that the certifying body mandates (and the end user wants to promote).
I agree, its not perfect, but it fits a niche for those clients who wish to ensure proper component use.
For mattf, since there may or may not be any specific environmental certifications for plastics, you might consider ISO 14001 as a basis for ensuring your company identifies and improves it's environmental impact.