The first is, like you said, that resistFingerprinting can be kind of a gateway to Tor in general, since Tor will do everything resistFingerprinting does, and better.
The second is that uplifting Tor features to "normal" browsers and allowing "normal" users to enable them makes it harder for website operators to say, "well, I don't need to worry about this because it's just Tor users and they're all criminals." Right now, enabling these features in Firefox will result in some website breakage, but as more people say, "well, this is a mainstream browser thing", maybe more website operators will start to accommodate the protections.
I think there's value in continuing to blur the line between Tor and other browsers, if only to push the idea that the kind of privacy protections Tor offers should be available to everyone across multiple browsers. Not to mention that it's nice to be able to take advantage of a few Tor features while still getting stuff like fast video streaming.
But agreed, there's definitely a continuum here, and it might be valuable for some people to explore farther down it.
(Or, if you’re just interested in helping advance the anti-tracking ecosystem! In which case you can test resistFingerprinting and file Webcompat issues when you encounter them — but be sure to mention that resistFingerprinting is enabled or your issues will probably be closed “unable to reproduce”.)