Sometimes you can't get a mortgage or insurance if the wiring is knob and tube.  So, disadvantage.

You can recycle the copper.  You can bypass the old fuses in such a way as to easily burn your house down.  Or screw in the wrong fuse in such a way as to burn your house down.  I'm pretty sure those aren't advantages...

Were you thinking there could possibly be a plus side to anything older than romex?

Sometimes it takes you a while to figure out what light switch goes to what.  The ones that are connected to an outlet will get me everytime.

Not many advantages beyond existence (as long as it's not aluminum). Lots of potential problems from worn out devices, rodent striped insulation, loose and corroded connections arcing (big fire hazard), lack of modern safety features (GFIs), etc.

of course the new wiring is going to better serve contemporary needs. but it's awfully hard to  spring for the cost and mess of replacing old wiring that's hidden behind plaster and lath if you don't intend to do a full renovation. our approach: pick and choose.

we replaced some knob and tube on an 80 year old adobe house last year at a cost of about 10k. we were lucky though because there was a crawl space that we utilized to bring the new wiring up into the walls for the switches. also it was a flat roof and there was a "sleeper roof'" construction so we also had room between the ceiling joist and the roof joist which allowed us to fish wires from the switches to the ceiling fixtures with minimal penetrations into the lath and plaster.

if not for these two advantages the cost would have been way higher, at least double.

Advantage: you can plug stuff in

Disadvantage: you can get shocked