As indicated above, if you are running out of no deco time on air well before you run out of air, Nitrox may be a big advantage in terms of extending bottom time.
However, Nitrox also gives you the option of diving Nitrox while using air tables or an air computer to provide an added degree of safety. This can be a big plus for older divers who may be getting farther and farther away from the parameters considered normal in most deco models. This can be particularly valuable on dive trips where you are doing repetitive dives over multiple days - a profile that pushes or exceed the prudent limits of most computers and tables regardless of age.
With regard to O2 exposure, there are 2 things to consider, your PPO2 and your total exposure (CNS clock). But in practice there is really not much to watch as you really have to push it and spend a fair amount of time in the water to approach the limits. And by then you will have long since had to start doing deco due to nitrogen loading.
From a practical standpoint, recreational divers doing no deco dives will find that nitrogen loading always imposes a limit long before the CNS clock is exceeded.
For example, if you plan to stay at or below a PO2 of 1.4 at depth, you can accummulate 150 minutes on a single dive and 180 minutes in a 24 hour period. (And this is actually pretty conservative as it is based on NOAA standards that do not give you credit for surface intervals.) So on 32% nitrox you could dive to 110 ft before you exceed a 1.4 PPO2 and stay for 25 minutes before you exceed the NDL. With 36% Nitrox, you could go to 95 ft and not exceed 1.4 and stay there for 40 minutes with no deco obligation. Consequently, in the worst case scenario you would have to do 4 dives to 95 ft for 40 minutes each on 36% nitrox in a 24 hour period before you would have to start worrying about the CNS clock. From a deco standpoint that is not possible as you will find you need deco stops to do the repetetive dives within the 24 hour period.
With a higher (and less conservative) PPO2 of 1.6 the limits get a little tighter, but not much as you are still allowed a maximum of 45 minutes for a single dive and 150 minutes total in 24 hours. So you can do a 130 ft dive on 32% for 15 minutes or you can use 36% to make a 106 ft dive for 30 minutes. But again you will exceed the no deco limits if you try to actually do five 30 minute dives to 110 ft within 24 hours.
So in practice, all you need to watch is the maximum operating depth for a given nitrox mix and the rest of the theory covered in the class on the CNS clock is just that - theoretical.