Simple terms:
Iridiums produce a better spark, but don't last nearly as long as platinums do, which is why platinums are more expensive. In terms of people not thinking that their spark plugs don't last as long as they are supposed to is due to the fact that spark plugs can be maitained just like everything else. When you take your spark plug out to look at it, and it is covered with black carbon, it doesn't mean that they aren't any good. Rubbing alcohol and a tooth brush is a simple solution to making spark plugs new again for all self assurance that they are still good. These websites below are helpful for you to pinpoint the status of your spark plugs, and what be done to prevent any other things that may affect the performance of you spark plug in the future.

http://www.motocross.com/motoprof/mo...lug2/plug2.htm
http://www.pdm-racing.com/features/plugs.html