What a wasted educational opportunity!
Piano arrangements generally exist so that those pianists who are absolutely obsessed with non-piano pieces (such as a favourite film theme, or a well-known pop song) can play it to their heart's content. If this process is removed, what is the point of learning the arrangement?
Working backwards from the arrangement allows the teacher to introduce the student to other repertoire and other instrumental combinations. Take for instance, all the numerous arrangements of 'Ode to Joy' - imagine playing the simplified piano versions without hearing some of Beethoven's exhilarating original work!
Understanding and knowing the original version allows the pianist to think about voicing (eg, making the most of the melody that was originally sung), tonal colour (eg imagining the original brass sounds or the cantabile strings), and style (eg through listening to original jazz recordings of the arranged version). These considerations can then play into the student's understanding and interpretation possibilities of 'pure' pianistic repertoire.