Hmmm well if you're actually smoking out of a PLASTIC bong then you should stop haha. What you're probably thinking of is an acrylic bong. The reason that glasses pieces are preferred is for several different reasons. The big one is that your average lighter does not produce a hot enough flame to melt or break down the glass so when you smoke you are only smoking the bud. Whereas metal, wooden, and acrylic pieces can give off some stuff that you really don't want to be inhaling. The taste you get with glass pieces is a lot cleaner too.