I am incline to agree with Henry about knob and tube wiring to a point. The fire hazard part can be had by abusing any type of electrical wiring including sheathed electrical cable,Bx,or conduit. I work with alot of knob and tube wiring. Henry stated that there is a risk of fire and increased shock hazard. This is not true for the average homeowner as the average homeowner does not touch the actual wiring. Or should I say should not be touching the actual wiring. The main reason you would have problems with knob and tube or even any wiring would be by abusing it. To large an incandescent light bulb in a cieling mounted fixture will cause dryness and cracking of any wire over an extended period of time whether it is knob and tube or sheathed electrical cable or even wiring in conduit. Also putting a space heater, which is made for casual operation and not to be run day and night will cause a receptacle to get hot and even at times burn up. Alot of problems with wiring are caused by rodents in attics and basements that by chewing on the insulation of the wire are sharpening their teeth and causing possible short potential. It is interesting to note that this rodent problem causes more damage to sheathed electrical cable wire than knob and tube because sheathed electrical cable type wire has all 3 conductors within about 1/8 of an inch of each other,(hot, neutral & ground) where as knob and tube wires are usually separated by a wood stud or even a separation of 16 inches. I guess I could go on and on about this because I work with so much of this type of wiring being a contractor in this field. I'm sure there are others who will disagree with my statements and usually those are the people who are in it to make lots of money telling people you should change something when it is not necessary. And they wonder why people don't want to call electricians.