To the above I would add that postermania is a direct result of the authentic assessment element of classroom pedagogy and is related to the idea that projects are much more descriptive of student learning than the oft maligned multiple choice test. Unfortunately, projects are time consuming and bulky to authentically assess, regardless of the complexity of the project, and since no one really can effectively evaluate student learning through most of the projects, posters are really easy to do, with little strain on the teacher, and if they are glitzy enough they make wonderful things to hang on the wall. After all, the modern view is that schools that are awash with evidence of real student work must be doing effective jobs of teaching.
I would submit that most of the posters I have seen can show quite clearly that teaching has taken place, but tend to be very difficult to prove that learning has taken place, especially when the posters are collaboratively developed and electronically researched and printed. The assessment is, in reality, neither effective nor based upon real problems but are mostly artificially done because the students have long since lost the basic skills with which to develop the real learning.