I've never used such a system, but here's the claimed advantage:
http://www.aquametrix.com/productnews.html
"60 Series Differential pH Probes
(Same probes are available in ORP)
All P60 pH probes use the differential pH measurement technique, proven over almost three decades in thousands of applications worldwide. This design utilizes two glass electrodes to measure the pH differentially to a third metal (ground) electrode. The second glass electrode is protected from the process by a double junction replaceable salt bridge.
The resulting true differential measurement probe has several advantages over conventional probes:
Ground loop problem eliminated: Ground loop problems are virtually eliminated by sharing a common a ground electrode.
Extended Life: The salt bridge is easily replaced (with needle nosed pliers), greatly extending the life of the probe. If the internal solution becomes contaminated, the probe can be rejuvenated at modest cost by replacing the salt bridge and reference solution.
Automatic temperature compensation: Automatic temperature compensation is accomplished through the use of an exposed thermistor at the tip of the probe. This thermistor placement provides rapid response for process temperature variations.
Transmits up to 3000 feet: The encapsulated preamplifier provides an output signal that can be transmitted up to 3000 feet over inexpensive cable.
Built-in 4-20 mA Transmitter available. The P65 version encapsulates a blind 4-20 mA two-wire transmitter that can be wired directly into a PLC. "
