I've changed spark plugs/igniters on everthing from F-16's to go carts and this is my professional opinion. I have seen plugs strip out the threads in aluminum heads. The official cause is somebody trying to remove the plug with the engine still hot. Let it cool down "completly" and use a small amount of anti-seize compound when installing the plugs and it will never happen. As far as how long to run a spark plug; the manual will not lead you wrong. On my rigs with out a distributor, I choose to pull plugs at 50k to inspect and regap them, then again at 100k to replace them. The only reason I pull mine at 50k is to keep the gap with-in specs; gap gets too large and it can cause undue stress or component failure in the ignition system. On my old 91 wrangler, with a distributor, I replace them at 30K-50K.