As I was installing a drive today, I made an observation about this topic.
From a maintenance perspective, NPN seems extremely counter-intuitive and makes equipment troubleshooting more difficult.
Normally when we think of something turning "on", we think of "voltage being present". If the headlight of your car isn't coming on, you're looking for +12VDC to ground at the bulb assembly. If a motor won't come on, you're looking for voltage phase-to-phase or phase-to-ground. If a mold heater isn't heating... well, you get the picture.
This also extends to physical sensors... let's say a float switch. When tank gets too low, turn on the pump. Doesn't matter whether the switch is NC or NO, the only thing that is going to start that pump is a "on" signal (full voltage) going to the pump from somewhere on the switch. Either through law, code, regulation, common sense, or whatever, the rule is is to switch "hot" and leave the other side("neutral","0V","negative") always on.
But if a PLC with NPN inputs has an issue with an input... you're looking for what? You stick your multimeter to the PLC input and a ground. What's this? No voltage? And the input is on? Is the PLC bad? Nope. The input is "on" and the voltage is "off". Get it? Yes, you can get around this by finding a positive wire somewhere and using it as a reference point instead of ground, but most people have alot of grounded cabinet and very few exposed positive terminals.
I think that this can frustrate many technicians especially as they have to trace problems from the "PLC side" to the "real world side" and have to constantly readjust their thinking. I suppose there are a few people out there who come from a strict "NPN" mindset who do not have to constantly make the readjustment, but I think they are a minority out of all industrial maintenance people.