They (AFCI) were originally required in bedrooms more or less like you noted, also things like lamp cords, etc. that get tightly folded over when a dresser is pushed against the wall. The original ones were actually AFCI and looked for series faults only. The newer CAFI (combination arc fault interrupter) looks for both series faults and parallel/shorts but ones that do not reach the breaker amperage, they are the ones required now AFAIK but AFCI became a generic term (might be important as actual AFCI only old stock may still be kicking around???), they will usually have different colour test buttons IME. Many argue that it is the breaker manufactures that are the conspiracy behind them to increase usage to make more money... The first AFCIs I used had a slight buzzing sound, CAFI ones now do not seem to.
Unless the latest Ontario code changed, they are required for all 20A or less 120V circuits except...smoke alarms, fridge and furnace as they do not want a trip causing a bigger issue.... I guess the NMSC feeding these is magic compared to all the other NMSC in the house.... some inconsistencies no doubt.
Many electricians/people hate them, IME it is mostly craft issues. In my limited experience:
- I have yet to have a power tool, including table saw, lawn mower, even my 120v welder or appliance trip one. (I am a SquareD QO guy, can't can't vouch for others).
- I connected my boiler and circulator pump just to "see" (to be truthful by mistake ), ran fine no trips but I switched it back of course...
- The only time I have had a trip I actually found a damaged piece of NMSC that was leaking current between hot and ground but far less than an amp. (A real PITA to troubleshoot this one, I did get lifted, but there was an actual fault and the CAFI protected the cabling not just the devices), a regular breaker totally ignored the fault. They do act like a poormans GFCI (a ground fault will eventually trip one but at higher current than 5ma, so don't use one as a GFCI!)
- IMO backstabbing outlets and loose marettes are the biggest cause of trips, maybe not day one but as the springs wear out, etc.. Use the screws on outlets, tight connections everywhere! I have also "heard" problems with using them on split duplex but I do not do that so I cannot say for sure.
When I did my rewire I pulled the permit a month before they became required to save money. My rewire got stalled for many reasons and I decided to eventually just go with them as per newer code, what is a grand or two compared to protecting the family.... did not have to.