5.2 V8 spark plugs

I recently took my wifes 98 GC into the local reair shop to diagnose a check engine light that was giving me a code which said cyl. 6 was misfiring. Prior to going to the shop I had replaced the plugs and determined that the ignition wires were firing properly. The new plugs were Bosch 4 point platinums. The repair shop which eventually diagnosed the problem as a leaking intake manifold gasket insisted that the platinum plugs were bad for the engine and replaced them with conventional plugs. I have never heard that "platinum plugs are not suitable for a distrubutor type ignition system". I have been running Bosch platinum plugs in my 2000 4.0 TJ for over a year. Any comments?? tia dw