There are definitely advantages to selecting an incumbent ERP vendor for your CRM software in terms of integration costs. However, a common problem I hear from SSPA members is while they love the newest version of CRM from SAP (or whomever), their ERP platform is using an old version and would need to be upgraded in order to use the newest version of CRM. The business users are typically given a 5-7-10 year time frame (!) before the ERP/supply chain/manufacturing system will be upgraded. In this situation they choose another CRM vendor (usually an OnDemand product so there is limited involvement from IT) and may loosely integrate to the SAP back office–or not integrate at all.
Also, if the ERP/supply chain/manufacturing system has been highly customized (as is normal), don’t expect the CRM module to be automatically integrated. You will likely have to do a lot of customization work in CRM to take advantage of your existing custom data and processes in ERP.
Bottom line, if you have a current version of SAP ERP, then adding on SAP CRM makes sense. Just don’t assume it is automatically easier and cheaper than a 3rd party CRM application unless you have deep integration requirements to the back office.