It depends on type of engagement. Some projects has been asking for very limited engagement, but the important point is regarding the evaluation of sustainable development projects. Without stakeholders participation and no feedback from them, projects would go nowhere.
Stakeholder engagement is an absolutely critical part of sustainable development. Proper identification of problem definition, what's fit for purpose, development of potential solutions, implementation and diffusion are all important roles for stakeholder engagement. Research that does this is generally referred to as transdisciplinary and requires that stakeholders and researchers work together.
After the research role, boundary organisations may continue to work with stakeholders to ensure that solutions continue to be disseminated adapted further. Reporting of the nature of engagement, along with outputs and outcomes helps close the information loop required in a consciously or deliberately adapting system. It also provide information for further investigations that may be needed down the track.
A colleague, Celeste Young, has provided a guide that discusses many of these issues based on her work and experience. You can find it here:
Book The problem solution framework, Process guidance for adaptat...
I agree with Roger. Essentially Stakeholders are crucial to the process of determining what is meant by sustainable development: i.e. what counts as sustainable in any given context. Also, researchers are not always effective implementers so stakeholders have a key role when it comes to implementing actions towards sustainable development.
I am in agreement with Roger and Thomas... Stakeholder engagement is critical to the success of sustainable (or any other type of) development. Without engagement your strategy lays idle. Process or quality improvement also suffers without stakeholder engagement -- particularly employee stakeholders.