Many high end agencies and trainers use airsoft in FOF and simulations. Been in quite a few and they are very useful. If you haven't run through them, then not recommending them isn't really valid.
The idea is that you have a thinking opponent as compared to a dummy (haha!). Having live fire against real people seems not to work well. They are cheaper than sims and less restricted.
Also, training by yourself or with videos is good but the airsoft shine with well scripted scenarios and opponents.
As far as skills, without searching - there was a Japanese young man who trained there with airsoft, came to the USA and did very well in real gun matches with a little real gun practice before the matches.
Square range practice is not a bad thing but insufficient. 3D targets are good for shooting at angles off straight on.