Isolation of analog inputs

Hi guys. My plant is currently planning a control system upgrade from DCS to a Controllogics PLC system. I've heard alot concerning the need to isolate the incoming 4-20mA signals from the field using isolation amplifiers before running the inputs into the analog input cards for the Controllogics. What advantage will this give me vs. using isolated analog input cards for the PLC? I'm trying to consider both the economic and functional differences.

Thanks,
rickboy25

A lot of installations don't use isolated analog IO at all.
I know it is very rare on sites I have worked for.
Like all of these methods, the importance is risk reduction. Decide what risks are involved, the probability of the risk, the possible consequences and the cost to fix the consequences as opposed to the cost of prevention.
For us, the worst credible problem is that someone will hook up straight 24V DC to the analog input. Usually this doesn't do any damage.

In the past, we had some 0 to 100V tachos that were run through a voltage divider to step it down to 0-10V. This needed an isolator since the likely consequences were 100V on the card.