Live worms need something to eat, and normally it's the decomposed (or decomposing) plant matter they go after. They break it down, and shit out the result, punching little holes all through your soil. Also, they don't eat living tissue, so your plants don't get harmed. Earth worms are a farmer's best friend.

I would doubt that even a single one would have enough to eat in anything smaller than a ten gallon drum, and if the soil dries out, they die.