OK, so it looks -- in my case -- like it's kind of a wash. My motive to buy the stuff myself, as opposed to the kit, was because I planned to (and did) use the greater amount of wire (that I obtained on my own) for other projects.
The Jazzmaster specific parts are below, as follows:
Slide Switch On-On: $6
Roller pot knobs (2): $12
Toggle: $12
1-Meg Linear roller pot: $4.50
50k Linear roller pot: $4.50
Roller pot mounting bracket: $10
Jack: $3
1-Meg pot (2): $10
Which comes to $62.
The wire cost me $7 for three spools, totaling 75-feet of wire.
That brings the grand total to $69. I'm excluding shipping (since I bought a lot of the parts from Warmoth at the same point I purchased my body and neck and, well, because the orders from Darren Riley and Stew-Mac included components for other projects).
The All-Parts kit provides you with seven feet of wire, total. Enough for your guitar, most likely, but with nothing useful to spare.
So, like I said, it seems to be a wash. They're saying all the components would cost $87 and that's simply untrue, but sourcing the parts yourself will cost you the same $70, except you'll get whichever make/model of components you chose, more wire if you want it, etc. etc.
Also, I recommend copper tape for the back of the pickguard. It made a big difference with shielding, for me. I didn't bother to shield the cavity, just the guard. Made grounding the whole thing very easy. It ain't cheap, but it provides enough for a bunch of builds.