Re: "they need to have a clear business case"
"Do any of these ERP systems actually bring business advantages that compensate for the high cost of installation, configuration and maintenance?"
Yes - assuming the migration to ERP is done well and the HR side of automating significant parts of business processes is handled well, there is scope for cost savings.
The challenge is how well your current business processes map to the chosen ERP solution and whether your requirements are stable/increasing/decreasing. The systems I have been involved with (as an on-looker, not directly involved) have allowed significant increases in volumes with minimal increases in staffing and associated costs such as office space which allowed companies to grow - if ERP had not been used, I doubt the growth would have occurred at such a high rate because they were eventually limited by office space etc but it was on the sales side rather than back office/support staff side.
Conversely, I have seen terrible business processes carried over to a new ERP system at a Council. The effort required to get the ERP system to conform to existing business processes was significant and within 2-3 years customers forced the council to drop the business processes (in-person payments) as they preferred alternative payment methods (bank transfer/telephone/web) and the majority of the staff were made redundant.
Most ERP systems fit somewhere in the middle of those two points - they add the ability to improve business processes IF a company is willing to change BUT most companies are unwilling to change large chunks of their business processes, do many of the processes to address something that is no longer an issue (i.e. fraud or other accuracy issues in paper-based systems or processes that require re-entering data multiple times) and hate the systems with a passion...