Almost everyone is missing the point of this circuit. It is a protection circuit for the ohms function of a DMM. In normal operation, no current flows through the transistors. As long as the normal ohms excitation voltage is present, the bipolar clamp is not conducting. The main selling point is the leakage current is so low, that in parallel with the unknown resistor, it does not cause any errors. If you used standard zeners, you'd never get a 20Megohm circuit to be accurate!
When an external voltage is applied to a DMM in the ohms mode, this is a fault that can damage the excitation circuitry, which has to be fairly low impedance. So a PTC device is placed in series with the excitation source. The dual transistor clamp will zener at around 6-7V (it does not matter how accurate it is) from the externally applied fault voltage. The clamp circuit will heat up, but so will the PTC. Once the PTC switches to its high impedance state, the clamp is safe, the DMM is safe. The clamp circuit does not have to dissipate power for long because the PTC switches.
This circuit, built with 3904 transistors, is proven reliable, and can protect the ohms circuitry for thousands of events without failure. There is no noise generated in normal operation. Noise generation does not matter when the circuit clamps, because the ohms function will overload. If you use two back to back, it is a bipolar clamp that protects the ohms circuitry from the accidental application of AC line voltage.
All that being said, you can use 3904s and similar transistors as low leakage diodes in a lot of circuits. Using just the BC junction, you can select devices that leak in the low pA range. This is very advantageous in circuits like sample and holds where even a low leakage diode would not suffice.
I do not suggest or recommend that you would ever use a forward biased BE junction as a zener. It would not be capable of much power. But given sufficient resistance in front of it, it is a great clamp...a low leakage diode in one direction, and a zener in the other. Emphasize clamp! Not a full time conducting zener.
A major advantage of this circuit is that it is dirt cheap!