Core ERP (finance, procurement, HR, payroll) should be part of one system. It creates too many issues if your split up this “integrated core” Extended functions however, which in the past may have been modules of an enterprise-wide system such as utility billing, permitting, recruiting, etc. can be (and in my opinion should be) evaluated separately. Many governments do go with a best of breed option for this. The reality in the market is that Core ERP vendors focus on core ERP functionality. Other best of breed vendors that focus on those extended functions can provide superior functionality. Interfaces to these projects is then not terribly difficult.