From Ecology Action
Earthworms in normal amounts are good for the soil.
However, vermi-composting, which is the pracice of using large numbers of earthworms to break down organic matter more quickly, is challenging because the worms tend to deplete organic matter unsustainably in their composting process and also make nutrients too available.
An alternative to vermi-composting is the composting method used in the GROW BIOINTENSIVE process, which allows the organic matter to break down at a more moderate rate, making the nutrients available more gradually. While worms can be a part of this process, they are present in smaller quantities, closer to that which would be seen in a natural setting.
For more information on successful composting methods, read Chapter 3 "The Use of Compost and Soil Fertility" in How to Grow More Vegetables.