OK, here is how it could cause a problem:
A 4-20 mA signal is an analog representation of a transducer. The transmitter is calibrated (zero and span) to represent the high and low range of operation for the particular process. For example, a CONTROLLED process that ranges from 1 psi to 100 psi would be 4 mA=1 psi 8mA=25 psi 12 mA=50 psi 16 mA=75 psi 20 mA=100 psi
There would be a setpoint somewhere in the middle of that range where the process is operating normally, perhaps 50 psi.
If a disruptive condition causes a pressure increase, say of 25 psi, the transducer would sense it and adjust its output to 16 mA. At the PLC, the algorithm of the microprocessor senses a deviation from its set point. It calculates an appropriate response and sends another signal via a seperate 4-20 loop (which is also calibrated) whose responsibility is to adjust a CONTROL VALVE. In this case, the control valve would want to open wider to relieve the pressure in the system. So, the steady control condition of 12 mA loop current would hold the valve actuator at 50% travel. The upset requires it to open, for simplicities sake lets say 25% to relieve the 25% increase in pressure. So, the valve opens to 75% travel, relieving the pressure and then eventually the whole system gets back to equillibrium.
OK, I explained the system to show you how clamping it could cause trouble. If, anywhere along the circuit, the transucer is bypassed or shunted, the entire loop could be thrown out of whack and the valve actuator could slam shut or be thrown full open, creating an overpressure condition or a dump. In a system with $100,000 or more of product or raw materials, or explosive, etc etc, this could be a big deal.
Also, these PLCs often have multiple loops operating off of a single supply. If one loop gets clamped and it effects the power supply voltage for all the loops, an entire process could be sent into chaos.
Generally speaking, these PLCs have many safeguards and back-ups systems, but the point is valid.
I'm just saying, and I'm not a Process or Electrical engineer, but I'm not sure its a good idea.