Street motorcycle brake rotors DO wear under normal usage. How long does it take for them to wear below the recommended minimal thickness varies, depending on several factors; how aggressively you use your brakes, how much weight is on the bike, mountain roads riding...etc, but most importantly, the pads you're using; aggressive pads will wear the rotors out a lot faster than softer ones.
Checking the rotors thickness with calipers only takes a couple of minutes and should be part of routine maintenance.
The minimum thickness stamped on the rotors by the manufacturer is a good indication that, as a wear item, they need to be replaced eventually to remain optimally efficient and safe.
Can you keep riding a bike with its front rotors worn thinner than recommended? Sure ...would the brakes still operate normally? Probably ...how far are you willing to go to find out the failing point of your main safety component?...
Just like you would replace tires long before the rubber is worn through to the cord... or not.