Sometimes one needs to power a circuit from one source or another.
Most LDOs, or indeed most normal regs, feed current back up to the source. LDOs tend to use a PMOS pass transistor which has a parasitic diode.
I am doing a design where I am using the Ricoh R1191 for this
It's not dirt cheap but not crazy-priced either.
I am wondering why this is rare. Is it not possible to make a PMOS device without the parasitic diode? Or have some other series element which gets turned off when there is no input? It reminds me of an active rectifier in switching power supplies, to avoid the Vf of the diode(s). There is even a circuit for a bridge rectifier, although that was commercially implemented with a complicated chip to drive the four gates, IIRC.
One obvious solution is to use a normal LDO and have a diode in series with the input, so long as you can be sure nothing funny will be hapenning inside with the ground lead which could still pass negative current.