If you're using a general purpose blocker (uBlock, uMatrix, PoliceMan, AdBlock, etc, even NoScript), it makes sense to actually disable the internal blocker (less hooks/rules to parse), as FF's list is not even remotely comparable to what you get by subscribing to a couple of community-maintained ones, plus there's no convenient way to tweak the rules (something that other addons excel at).
FF internal tracking protection is somewhat nice for the casual user, and it's going to stir some extra polemics about content blocking (which I consider a positive thing), but it's nowhere as effective as the others. I fear it's also going to be circumvented more quickly, promoting more inline JS, supercookies and fingerprinting techniques.
Overall, it's not something that I would have included in FF from a purely pragmatical perspective. It's just opening Mozilla to direct liability, while not providing anything for the privacy conscious person.
I would guess that there are many people who use Private Browsing mode that don't understand or use security addons. This change may not help many Hacker News users, almost all of whom do understand and can easily use the addons, but we are only a tiny fraction of the Internet.