Welcome to CPF carmel!
I've not had an MN20 or MN21 die of old age. I've put lots of batteries through my two M6's. I don't know whether I've put 20 hours through an MN21 yet. That's a lot of batteries and a lot of use of a very bright flashlight!
In my opinion, the lamp life is not an issue. I believe in having spares but for SureFires, I don't believe I need to have spares. If I was getting an M6 I would get a spare MN20 and an SC2 for the MN20, and at least 72 SF123As and a BlackHawk M6 pouch for the M6.
If you are going to be using the M6 (MN21) as a tool then having a spare would be a good idea. If you are going to be using it for fun then it is not as important as batteries or carry method in my opinion.
For most people (who don't use the M6 or M500B in training/use on a regular basis), the lamps should last for years. If you drop the M6 a lot then the bulb will weaken and can die.
Finally, to answer you question (!), in my experience it is not normal for the bulb life to be less for SureFire lamps which have high outputs. There is no evidence that I've seen that suggests that high output SureFire lamps have shorter life-spans. When you get your M6, I believe you will be more concerned that it costs about $7.50 to power it for about 20 minutes using the MN21. Of course I can think of ways of spending far more than that for 20 minutes of fun. Going to the pub at lunchtime for example.
I hope you enjoy your M6!
Al /ubbthreads/images/graemlins/grin.gif
