Again, you have outlined the issue most eloquently, Donna. Studies show that providing extra time to students without disabilities on a test that is designed to be finished in a given amount of time does not significantly improve the performance of the student (as you say -- if you don't know the answer, all the time in the world won't change that). But providing extended time for the student with a learning disability CAN significantly improve their performance. Accommodations always focus on "each according to their need" and these students NEED more time to show the same amount of mastery.
Dr. Jane Jarrow