ERP Essential For a Small Business?

There's a correlation between complexity and needing an ERP. Most small businesses don't need one, and using one would be a waste of recources and time.

ERP becomes essential as a business grows and its day-to-day running becomes increasingly complex. Small businesses don't really require ERP because their business operations are more manageable, while ERP comes at a high upfront cost and involves a learning curve. 

As a former IT Consutant I would actually argue ERP is far more essential for a large business than a small one. The overhead involved with a large ERP system for a Small Business often doesn't make sense when associated with revenues/volumes.