PKJ is too risky and low reward. You cancel a forward dj (already a big mistake with Ness, Ness' dj is absolutely sacred for Ness) with PKF to gain huge foward momentum boost so you land right next to your opponent while the PKF is on them. It sounds good on paper but it has many issues. One of the issues is that the PKF can simply be shielded and since Ness has large endlag frames on his PKF aerial landing (PKJ can't be PKF aerial lag canceled unfortunately) the opponent will be free to counter with almost ANYTHING as Ness is helplessly stuck in his endlag frames. Even if the PKF hits an opponent the PKF pillar is ridiculously easy to DI out of so that makes this risky maneuver pointless (try this on a competent Marth and you'll get countered, jabbed, or faired everytime) since your opponent will most likely DI out by the time your aerial PKF endlag is gone and may still have enough time to counterattack you (depends on the character).
I believe the risk/ reward factor depends entirely on who uses it against what character.! You can do this during almost any part of your jump, and if done CORRECTLY you will always have options to react befor landing. You can drop under a platform and immediately do it, you can walk off of any platform and immediately do it. Of course I agree that it still is an aerial pkfire, and if you don't perfect the technique, you WILL get punished. but check this scenario out for example. I was playing this snake, and I took off his first stock. Upon respawn, I evaded him until his invincibility frames were up, and (because I hadn't done this before) I caught him off guard by jumping from the ledge and using pkfire as my momentum moved me away. He was hit with the firstpillar of fire, and since it his him above the ground he was undable to shield out of it. He attempted to DI away from me and out of the fire, which was a normal reaction for someone caught in the move. However, I used pkjump to not only get back on the stage, but hit him with a second pillar and hit him with uair. the two pillars held him in place so I could then proceed to using utilt and then another uair.
Now I'm not sure what theory crafting is, but if what I just did was it, then this theory was tested and match-viable. it racked up around 50% and I'm sure we can all be creative on how we set up pkjump. I believe it's superior to an aerial pkfire because you can put yourself at different areas of the stage depending on when you do it.
One thing I can agree on 100% is that it's
REALLY RISKY!! but if you know what you're doing and correctly use it in a situation when your opponent won't expect it, you can still come out with whatever reward you can successfully execute.
It's still pkfire, so of course the average competent opponent can still use the tactics they would usually use to get out of the fire (which is why I suggest hitting them with it so they're above the ground in it, less options for them and you can get a nice read if you're clever about it).
being safe, you can nair, fair, uair, bair, pkthunder, pkmagnet, and airdodge before hitting the ground if you use it correctly (sometimes depending on the stage, but they're all legit in some form or fashion). So almost every defensive tool is at your disposal, especially when you're close enough to the ground to follow up with the defensive aggro maneuver of your choice.
This is obviously not NEEDED at all whatsoever, but as a person who uses it, I recommend using it when the occassion strikes, because the risk/reward factor (deending on how much you've used it, like any more) will depend on when you use it.