If the remote controller is always hot, all it takes is an erroneous signal to trigger the winch. This can happen by bumping the switch, having someone in your vehicle bump the switch (pets, blonds, or whatever), something behind the dash contacting switch posts, the wiring anywhere in the circuit shorting or crossing providing power when not wanted. Since the winch itself is hot-wired, having the controls also constantly hot is a safety issue.
"Failure" in this context is not the inability of the winch to function, but inappropriate activation of the winch. Using an arming switch in circuit is functionally the same as disconnecting the stock controller. I suppose the stock contacts in the solenoid block could be shorted as well, but having it happen accidentally is very unlikely. So, the argument is only one of perspective. I use an arming switch at both remote locations because I have, more often than I like to admit, accidentally bumped the control switches.