Hi Thomas,
I don't really understand your question. Are you asking about CRM or ERP? IS-U is an industry solution based on SAP ERP while in SAP CRM there are some utilities specific functions.
If you are asking about IS-U the most obvious side effect will be additional license costs 
. If you are asking about CRM the main problem will be that most of the utilities specific extensions to SAP CRM depend on the availability of a SAP IS-U system (connected via RFC and the CRM Middleware). If you don't have a connection to an IS-U system most of the features won't work.
Christian