you have to replace them every few months whereas a metal muffler most last for a long time. there also illegal to have on a truck so why not jsut get a real muffler and not get a ticket? besides most mufflers sound better then glasspacks anyway.
i had a cherry bomb on my old TBI, and couldnt tell a difference from the stock muffler i replaced it with. you'll have to replace a glasspack every few months, and in my opinion, they sound ricey and high pitched. id spend the money on a borla or a flowmaster, you'll save money because you wont have to constantly replace it.