No-Rescue was solely (?) given as an option to stop unwanted PC's from rescuing another player. However, no-rescue also stops pets from rescuing. For instance, with my current necromancer, I do not want PC's from rescuing me but I want my pets to be able to. Would you consider this to be functionality that wasn't intended?

Not especially, no. It's a useful option, but not necessarily one without price.

If you did draw a distinction between pcs and npcs, you then have to consider npcs like angels that you may, or may not, want to allow to rescue you (since when some rescue you they get themselves killed pretty fast).