Big Ohmic Problem ???

What you do not want is any alternate low impedance path to ground via _any_ sense wiring

My feeling is that it should not be needed, except in the event of component failure
(which seems unlikely)

Yes

Sure but the choice should be made on the basis of likelyhood of failure

Its possible that the power supply 24V side could get a low impedance short to ground
but its quite unlikely.

I don't think the table ground can be looked at in isolation as being part of ohmic sensing. Wiring a star ground point should be done as well. That is really a seperate topic and would look after all the ground connections including the one added to the diagram.

There is a topic in the plasma section about wiring and EMI that covers it in detail.

Here it is forum.linuxcnc.org/plasma-laser/36734-le...ring-and-emi?start=0

Just one last comment on the fuses and then I rest my case:

Anytime you attach a small gauge wire to a high energy potential you have a possibility of fire due to thermal overload in case of a fault. That's why in electrical installations you are required to fuse the wiring according to the possible short circuit current in case of a fault.
In this case I would agree that a ground fault in the isolated power supply is unlikely. IF you use a double insulated supply that is.
Here you need to have two faults. An unwanted ground loop in the sense wiring and a bad ground at the table or work piece. As we have already seen this can happen due to a misunderstanding of the circuit diagram and forgetting to attach the ground clamp.
I'm not a safety freak but there was already at least one case of fire and another where the potential was very real by grounding the sense loop. So clearly some thought should be given to protection.