Today, nearly every business taking ERP implementation under consideration will have systems already in place that could be changed with newer ERP modules currently under development.
Consequently, a crucial factor in the adoption of an ERP system is identifying which current systems will be replaced, which ones need to be integrated, and which are performing well independently and should remain unchanged.
Keep in mind, the more data integrated into the ERP, the greater the value of the investment, so it makes more sense to integrate all systems rather than leaving them out of the ERP.