Re: In terms of farmers -- Drive a tractor??
Ah yes, the tractor stuck in the field. Been there, not sadly a combine harvester, but certainly largish tractors, the ability to get it going again is hideously important, and is becoming apparent to the folks here. The ability to replace a belt, change a pulley, replace a blade in the field is paramount. Lost time is very much lost money, especially for simple repairs.
What we're seeing is the electronic systems of these machines becoming more and more integrated. Raise the deck, need a sensor for that. If that (usually non-redundant) sensor malfunctions (and it's a difficult environment for sensors here) the whole machine stops. As Brian correctly points out having a machine out of action because of a sensor that cannot be overridden when it goes faulty to at least get the job done before it rains is disasterous.
Our local Kubota dealer at least still has a decent collection of spares, but I dread the day of just in time or on demand parts. When an delivery time is usually 3 days or more that does seriously affect productivity.
To give an analogy, imagine if a fan speed sensor dies in your server. The machine says "faulty" and shuts down, or says nothing, just shuts down and you need a proprietary piece of diagnostic equipment to read the fault code. To get the machine diagnosed and repaired you have to disconnect the server and take it to the dealer (at your expense) where they will hook it up to their diagnostic machine, say that part 213-81672349B needs to be replaced. It will take 3 to 5 days for the part to arrive and then they replace it and you can take your server back (again, at your expense) and then re-install it.
The solution is, naturally, to have on-call service professionals who will turn up on site and diagnose (and possibly order the spare part) and then replace it. The farmer meanwhile has to switch to the failover machine. This changes the economics of these machines completely.