I would think if they would seize in the steel pulley the belt can still run on a seized pulley but it will cause it to smoke should give enough time to pull over and shut the engine off or at least the driver should be able to smell the rubber from the belt.
I still personally feel steel pulley is better then plastic ones I'm sure it will fail at some point everything dose but I still feel that I could get away with changing the timing belt and water pump and leave the pulleys rather then replace them every time as long as they get checked to make sure they still function ok then I see no problem using them.
I changed a timing belt on my friends '96 dodge neon in October 2015 for the first time in the cars life it got a new water pump and timing belt and the steel tensioners where still working perfect so they were reused and the timing belt never even stretched the only reason why it was even changed was the water pump was leaking.
He's had the car since new and has done virtually no maintenance on it what's so ever it's still running with the original spark plugs and wires there's not much left of the electrode any more but it still runs and drives.