Ease of maintenance is not the driving force....
You are absolutely right there, but take two machine examples here.
One uses safeties with two sets of contacts. Both are series contacts. One side is 24VDC and used to pull in a safety relay. The other side is 24VDC and goes into a PLC input (yes that is right, you would think the AC side would be used to pull in the relay and the 24VDC would go to the PLC input, but not the case). The relay cuts off power to the output modules, and sends an input to the PLC preventing the machine from resetting. The other side does the same thing.
Now take a German machine (of course designed to European specs). It uses a safety PLC and all safeties have inverters that have to come back and make a circuit board happy, which then alters a signal and gives that signal to the input of a safety PLC. Much more difficult to troubleshoot with a meter because of the signal inversion, but no real way of knowing which safety is bad in either.
In the first example, it was possible to check voltages at the control cabinet and jumper out the bad safety (big no-no). Both these machines can really hurt someone.
In the second, you needed a dummy switch (which was kept as spare parts), but it could be done, though I never brought that subject up to anyone after one of the techs was not able to jump out a safety circuit. I didn't want anyone jumping out safeties on these machines as it was incredibly dangerous in in case 1 could cause massive damage to the machine.
Rather I just showed the techs how to use a known good switch to quickly determine which switch was bad. Now, was the second, much more complex and difficult to troubleshoot method of safety really necessary if both could be bypassed quite easily?
