I'm an RAC patrol, and I've lost count how many times I've been told "it's only the fan belt mate" only to find the alternator seized or a tensioner hanging off.
As much as I'd like to have a part for every car in the back of my van I'd need the Tardis model to carry them.
Once I did collect a part en route to a job on the insistance that the member who claimed to be a mechanic was adamant the belt was the only fault.
On arrival I found the PAS pump pulley hanging off and when I asked him for the £15:20 for the belt which I purchased out of my own pocket he refused to pay for it as it won't fix the car (although his old one was knackered).
It's still in the back of my van a year later, won't be doing that again!
As for sending the correct vehicle first time, if we send a patrol we get "I asked for a truck( sorry but were not going to recover you 200+ miles 'cause you never had your spare tyre repaired and you can get a part used one for £10 at home).
Or we send a truck and we get "I asked for a patrol as I have a spare cambelt in the boot for my V6 Vectra"
Guy's I don't claim were perfect but I do believe were the best as I've worked for the AA,Landrover Assistance (Mondial), and now the RAC
and we genuinely try to do the best for the member with the resources available at the time you breakdown.