There is a Positive and Negative Speaker wire for all the speakers in your Nissan.. You can find them in the doors.. Or, usually easier just behind the headunit. The headunit Factory or not.. Factory Bose Speakers that are self amped or not.. etc.. Require a signal from the headunit. So you have the Front right, Front left.. and so on all right there in that wiring harness area you already put the new stereo into.
Some people will tell you that you need to run ALL new larger wire and NOT use any factory speaker wire. That's crap. I was going to not use ANY factory wiring in my Nissan (it's a similar setup even if it's a Nissan 350Z) -- Then I finally wised up (thanks to the nudging of people on here) _- And I ran new speaker wire from my 5 channel amp under the panels/carpet to the back of the headunit.. spliced into the factory wiring for the Front speakers there -- And my rear speakers it was just easier to run new wire to (but that's just the way my car was designed) --
Point is,.. you can use all the wiring already in the car.. even amped with aftermarket speakers and it will not only work, but sound just fine, if not great.
You don't want to push 1000watts RMS through 24 gauge factory wiring (ie: A high power sub).. but some nice $100-$200ish components that run 50-80watts RMS will work just fine with factory wiring (and safely). I Push 70watts rms X 4 speakers for my mids/highs.. and 300watts x 1 for my 10" sub... (In my setup I only have the front speakers using part of the factory wiring.. but even if I didn't run new wire to the rears .. they'd sound just as good..). I do recommend running dedicated wire to your sub.. (if you even choose to buy one).
I can't tell you exactly how to take your Pathfinder apart and so on -- but Nissans are all pretty much designed the same with wiring (give or take) after X year with regards to the stereo/speakers, etc..
So this may help you:
http://www.installdr.com/Harnesses/Nissan-Wiring.pdf
Bottom line: To answer your question -- Yes, it makes sense, and Yes you can do whatever you want with that factory wiring. Obviously you can't use the Bose amps on aftermarket speakers .. and so on.. but you can use an aftermarket stereo (I assume you put in) -- with a set of aftermarket speakers (just powering off the headunit) (which will be something like 10-20watts RMS per speaker at best..) -- OR go bigger/better with higher wattage and quality speakers and grab an Amp to power them, and run RCA's from your headunit RCA pre-outs to the Amp, and then of course the Amp goes to your speakers.
Some people just want two $50 speakers in their doors,.. some want $15,000 worth of speakers.. I understand and agree with it all.
Good luck with it,