1. Stop wasting your time on YouTube: the comments are a manifestation of the dumbest people on the internet.
2. As Mr.Genius himself pointed out, "I always use metal BB's... They hurt my friends more when we fight with our guns" (he sounds like he's about 12, too)
3. Most AS inner barrels are brass, or brass or aluminum with a coating. If you shoot a BB made of a harder material than the barrel, guess what happens?
4. For safety. Plastic BBs will flex/deform/break on impact -- and just like a racecar flying apart during a crash, the energy goes into that. A silica or metal BB doesn't deform so the energy is passed straight into whatever it's hitting.