I've never seen a cam fail in normal or heavy usage. I have only seen one case in a specific set of conditions while intentionally firing to destruction for testing where the cam integrity was compromised. It is possible under some specific cirumstances for the firing pin to fuse to the cam or the cam to fuse to the bolt or bolt carrier but thats talking about heat values and forces way beyond what you will ever encounter and if you did encounter them you will not have survived anyway.
Its possible for a broken firing pin or bolt, in some cases a bolt carrier, to dig into the metal of the cam in which case the cam will probably still work but its better to replace it if the stuff dug in beyond small simple light scratches or scarring. Only seen that happen one time in going on 39 years with this weapon platform. Broken firing pin put a small scar on the cam, it was smoothed over with some sand paper and kept in service without any effects but ended up being replaced anyway in normal maintenance.
If it comes to the point where a cam is stressed or compromised so severely that it will fail, its more likely the bolt, bolt carrier, firing pin, or those things in the lower, will have been destroyed or compromised in some manner first and the cam will have survived intact. Its most likely the weapon will have been damaged beyond the point where a small parts replacement would return the weapon to service before a cam is compromised beyond use.
My suggestion is this, to have on hand; Get a lower parts kit that contains everything in the lower (probably never use them all, but ya never know and the kits are reasonably priced), a complete bolt/bolt carrier with firing pin for fast replacement if need be (moderate to heavy lubed - maintain heavy lube to help make sure for emergency needs replacement - and ready to go of course), a few sets of gas rings, a few firing pin retaining pins in case you lose one during stripping down the BCG (hate it when that happens), and a spare charging handle.
