Comment by AStonesThrow

Someone who is neither contributing nor necessary to a meeting may still be required to attend the meeting. For example, a mandatory training meeting includes people who are being trained, who are in this category.

If the meeting fails to accomplish its objectives in 50 minutes, then participants may excuse themselves with a clear conscience, but they may find themselves less-informed than coworkers who chose to stay for the entire session. Especially if there is "Q&A" for clarifications at the end of it.