SMS 2003 e SCCM 2012 - Coexistence

Both can coexist and note that in ConfigMgr 2012 you can use a "boundary group" for site assignment and content location therefore this help to avoid overlapping boundaries for site assignment because we can use one set of boundary group for site assignment and another set for content location. Read more
So this make possible to have SMS 2003 or ConfigMgr 2007 boundaries and ConfigMgr 2012 boundary Group used for content location overlapping.

I've done many installations before but this time I'm stuck. I've created the container and give it the full permissions for the site server machine account(including all descendant objects) but SCCM 2012 still cannot create the necessary objects in it. According to hman.log system management container exists but it cannot write to it. AD schema hasn't been extended. My customer is hesitant for now, so we plan to extend it later. And afaik without extending the schema SCCM can also operate without issues. Any idea with the issue?

Publishing to AD without having the schema extended does not make sense. ConfigMgr can work without publishing to AD, but it's easier if the schema is extended and publishing is active. Why is the customer hesitant?