Tom beat me to it.
AS does not needto be replaced. After its nutrients are exhausted, it just becomes a substrate like any other. Sure, eventually it starts to break down into sediment, but then it's comparable to MTS.
You can grow any plant you want without AS. AS just makes it a little easier.
And XanAvaloni, just FYI, buying AS from a brick and mortar store is always about the same price as having it shipped to you from ADA-NA or ADG. They factor in shipping when pricing it for online sale. ~$36 total for 9L is what you pay either pick-up or shipped.
Is it worth it? Well, that's just a matter of opinion...how much is $40 to you? If you can afford it, it's worth it. If you can't, it's not.