Deploying Middleware Data and Application Data to PDBs in Same CDB

Our company's customer is insisting that we use a single container database to host the middleware metadata as well as our application's large data sets.  One container database on one server with separate PDBs for the various data sets.  My question is whether this is an advisable approach.  As a DBA, I have always insisted that the middleware metadata be hosted in a separate database on a separate server in a separate security zone, but our customer wants everything to be combined into a single container database.  Their argument is that this is the whole purpose of containerization.  Maybe I