A meaning of life essay would give the students an opportunity to portray a clear understanding to his or her professor what they will have learned throughout the process of the class. This would allow students to put into their own words the perception of what was taught to him or her without setting to many guidelines.
A meaning of life essay can be very helpful to a professor in finding out what his students have learned and if he properly taught them. I think it will outline what they took away from the class and what they think of the religions and how it affects the life around us.
I have been a professor of religious studies and unless one is trying to calculate the learning around one particular religious tradition, it is helpful to have a student write an essay on the meaning of life using highlights from the particular religious and spiritual traditions that have been covered.
A meaning of life essay is not a helpful way for a professor to assess student learning. Professors tend to be influenced by the students religions beliefs and might grade the paper by the meaning of the student's religion, instead of the quality of the paper. This type essay does not need to be written.
People need a real metric for determining whether a person has learned something, and a single essay about a person's life is purely subjective and at the whim of a professor. It may be a useful part of the curriculum as a whole, but should not be the sole metric used.