IMHO, this is not an issue of 'grandfathering'; 480V ungrounded deltas can be compliant with current code.
The questions are:
Does it offer benefit that outweighs the risks?
If the ground detector lights indicate a problem, is there someone keeping an eye on things?
How expensive would it be to retrofit a grounded service?
IMHO if they don't need the continuity of service benefits of an ungrounded system, and if they aren't _required_ to have an ungrounded system (eg cranes over combustible materials), then I'd push for a grounded system.
-Jon
