"Inert" and "noble" ar two completely different things. Inert gasses are gasses that when we breth them in, our body dosent use them and we simply exhale them out (such as niterogen, and actualy all the other gasses but oxigen, that are not toxic). "Noble" gasses, on the other hand, are gasses that do not react CHEMICALLY with other materials, wich makes them, ofcourse, also "inert".
As for narcotics- from experience, you do not get as narced with niterox, as you would with air. Oxigen might be as narcotic as niterogen, but either since our body uses the oxigen or the effects are not cumulative we do not get as narced. Besides, I belive that with oxigen, the toxicity is differnt, and not narcotic. It leads to seisures and other maladies, reffered to as Oxigen Poisoning, which is completly different from nitrogen narcosis.
As for other gasses: All gasses are actualy poisenous in such or other degree. All gasses have certain affects that they couse when used at higher partial pressures than normal. Effects of different gasses are not cumulative, and each has it's own effect.
Helium couses HPNS at higher partial pressures, which is a kind of wierd seisures, and get's the person very upset (heard it from people that experienced that when working with heliox at 200 M deep). When used at more "conventional" depths helium is usualy free of toxic effects, but it does conduct heat faster than other gasses, so it makes you cold faster.
Another problem with heliox/trimix is the cost. filling a tank with these will cost about 50$, whille air or niterox will cost much less (4-7$). So when diving to depths that do not require it, there's no reason to use it.
