I just finished my Nitrox classroom, so the stuff is fresh in my mind.
PADI indicates that diving Nitrox does NOT provide additional safety, however this is NOT based on anything but incidents of DCS in air vs. Nitrox. Both are low, and as the numbers are simply a good guess comparing the number of dives vs. DCS incidents, and they do not take into account individual profiles or multiple dives, I'm not sure if the conclusion is valid (in fact I think it is not, and most instructors I've discussed this with including our Enriched Air instructor agree).
Take a look at Nitrogen intake. Using the PADI RDP for air, after a 60 ft dive for 39 minutes one is in the pressure group of P and it would require a SI of 2:28 to get back to an A pressure group. Using EANx32 doing the same dive one is in a pressure group of K and an SI of 2:05 to get back to A. The NDL for a 60ft dive is 55 minutes on air, and 90 minutes on EANx32.
It would seem to me that if one absorbs less Nitrogen during a dive, there is less chance of DCS, and the numbers and limits when comparing air vs. Nitrox make a very compelling argument. If one were to push the NDL on air, and hit a 60ft dive for 55 minutes you are pushing the limit. If one were to dive the same profile on EANx32 one ends up in pressure group P which is conservitive. How is that not MUCH safer?
I realize that there have been no studies done (for obvious reasons) on DCS doing air vs. enriched air. However I can not see how anyone can ignore the fact that diving enriched air is not MUCH safer. The bottom line is that on EANx32, most dives will NEVER hit their NDL with any sized tank because they will run out of air long before hitting the NDL. With Air, that is just not the case.
As for the P02 limits, once one starts doing the math, it becomes clear that it is next to impossible to hit the limit during even a five dive day. For example if all dives are done to 60 feet max with a PP of .9 ata, one would have to do 360 minutes or SIX HOURS of diving on EANx32 to hit the exposure limit.
The only real risk is O2 toxicity, so just don't dive deeper than the mix allows, and all is well.
For $100, IMO it's worth getting the certification, and as I am doing exactly that, my money is firmly where my mouth is. The air fills are also more expensive, so many divers choose to do Nitrox on deeper dives, and go to air on the second.