I can understand plastic for a 2,5" (350 g-shock), but not for 3,5" (70 g-shock)Probably a bit of both (design and protection). I would also suggest cost as a factor, plastic is less expensive than more durable materials.
I suppose it depends on the actual internal mounting of the HDD and the density/type of plastic being used for the enclosure.I can understand plastic for a 2,5" (350 g-shock), but not for 3,5" (70 g-shock)