Online postgraduate courses for professionals often use discussion forums to promote engagement and interaction. Equivalency theorem suggests that student-student interaction may increase satisfaction but is not necessary for achieving desired learning outcomes. Therefore, costs, as well as benefits, should be ascertained. We used data from student feedback and interviews to assess the perceptions of part-time postgraduate distance learners, and analyze their views of the role, benefits, and drawbacks of discussion forums. The aim was to assess forum efficacy in the context of the specific needs of these learners, to inform forum use and design. Thematic analysis revealed complex interactions between student context and experience, forum design and management. Structurally tweaking forums to control engagement may be particularly ineffective, stimulating unhelpful grade-focused participation and highlighting forum opportunity costs. The study revealed the importance of designing and managing forums, with direct reference to their costs and benefits for specific student groups.