I don't think the benchmarking that's needed is to check whether pruning unnecessary joins is helpful. Obviously it's going to be hard to measure on simple queries and small tables. But the resulting plan is unambiguously superior and in more complex cases could extra i/o.
The benchmarking people were looking for in the past was testing the impact of the extra planning work in cases where it doesn't end up being applied. I'm not sure what the worst case is, perhaps a many-way self-join where the join clauses are not suitable for pruning?
