From my experience, I don't think the reverse-protection diode is much of an issue. I can't
remember seeing any actual boards with those diodes on there, but have seen dozens or 100s
of examples without them, and with no doubt 10s or 100s of 1000s of those boards in the field,
including my own.
The datasheets below indicate there is only a real problem if the input to the v.reg is shorted or
instantaneously crowbarred to ground, and you have a rather large cap on the output terminal.
Few of us ever use crowbar circuits, especially ones with 10-20A capability, and how often do
we short the battery/etc straight to ground to remove power [think: big spark, fried battery
wires, exploded battery], as opposed to simply unplugging it, and which isn't going to produce
an instantaneous 10A crowbar to ground. Also, I'd bet the vast majority of halfway decent
designs are going to have an electrolytic cap on the v.reg input terminal, which will limit the
Vin discharge rate right there.
ref LM7805 d/s:
reverse-bias protection
Occasionally, the input voltage to the regulator can collapse faster than the output voltage. This can occur, for example, when the input supply is crowbarred during an output overvoltage condition. If the output voltage is greater than approximately 7 V, the emitter-base junction of the series-pass element (internal or external) could break down and be damaged. To prevent this, a diode shunt can be used as shown in Figure 7.
ref LM1117 d/s:
When a output capacitor is connected to a regulator and the input is shorted to ground, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage of the regulator, and rate of decrease of VIN. In the LM1117 regulators, the internal diode between the output and input pins can withstand microsecond surge currents of 10A to 20A.
With an extremely large output capacitor (?1000 ?F), and with input instantaneously shorted to ground, the regulator could be damaged. In this case, an external diode is recommended between the output and input pins to protect the regulator, as shown in Figure 4.
ref NCP1117 d/s:
The NCP1117 family has two internal low impedance diode paths that normally do not require protection when used in the typical regulator applications. The first path connects between Vout and Vin, and it can withstand a peak surge current of about 15 A. Normal cycling of Vin cannot generate a current surge of this magnitude. Only when Vin is shorted or crowbarred to ground and Cout is greater than 50 uF, it becomes possible for device damage to occur. Under these conditions, diode D1 is required to protect the device.