That said, would learning C++ add to my [market] value as a developer, and/or would it allow me to do things I otherwise could not with my current set of languages? If not, is there any reason at all why I should consider C++?
When the going gets tough, the tough reach for their C/C++ compilers. :)
C++ is seeing a revival of sorts due to IoT/Automotive/Fintech software etc. So from a career perspective it would be a good investment of time (if, for instance, you are inclined to work in those fields).
However, keep in mind what is being called Modern C++ (c++11 and later) is a completely different beast than C++ which is uttered in the C/C++ sense.
My prediction is that the reality of C++ in the near future would be a mishmash of old and new C++ ...and it would go the Java way -- heavily adopted but a lot of bad code written by confused practitioners.
Someone who writes a higher level language (like python, php or ruby) and can understand the internals (written in C not C++) can at times have a distinct advantage.
Not only are you able to/capable of understanding what is going on at a deeper level you now have the ability to optimize things in a way that would not normally be possible. On the PHP side I have used C to do some low level optimization that would not have been possible otherwise, and a long time ago simply get some features to work properly.
On the other hand, it's also a massive language, and modern C++ is very different than the legacy code bases you'll encounter in the real world. So it's a large time commitment to learn it well enough to code well in anger.
If you have the time try skimming a tutorial, and doing some exercises, enough that you can get the differences in mindset. But it's probably not the most efficient way to increase earnings, if that's all you care about.
Also, don't focus on your "market value", focus on becoming a better developer. You're a craftsman, not a piece of merchandise - have some pride in your craft. (not meaning to say you don't, I just liked the sound of that)