"It Depends" on the type of DBA that you're looking to hire.  Although I agree that performance optimization is incredibly important for the system, it may have absolutely nothing to do with what the job description is for the "DBA" position.  You also need to define what "performance optimization" is.  Some think that it's setting up SQL Server with the right Trace Flags, the correct number of TempDB files, and deciding which type of RAID should be used for which drives and possibly include some form of DR replication that keeps the RPO very high and the RTO very low.  Others think that it's finding and fixing performance challenged code.  Some include the "pipe" itself.