Common Law Employment Contracts for Small Business

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Does anyone have direct experience either working for a small business (less than 15 ful-time equivalent staff members) or know of a small business that has elected to continue using their common law contracts of employment, rather than moving to an Enterprise Agreement?

As long as such contracts were written in accordance with the Fair Work Act 2009 and other relevant awards, what would be some of the disadvantages for businesses of this size to remain using common law contracts?