i think you have hit on the main selling point and william did too. it seems about ease of access to regular competition. i like the idea of the points system altho it occasionaly makes you (the driver) a victim. i currently have a free 3 month trial. i like the regular racing it offers. at the same time,
Those are the main selling points of iRacing - "no hassle" access to simracing, a points system rewarding good, clean driving (though bad behaviour abounds there too), leagues, championships with other fellow racers and the possibility of watching and even racing against some real world professional drivers.
On the other hand, physics have too often ups and downs, given the fact they're working on improving it. What might work one week may not work the next week for this or that car, and that is a factor for some - especially considering the work being done in rFactor, GSC, the upcoming rF2, AC and RRE.
Server problems, being down for a whole day or parts of a day in consecutive days (there is a thread about this at NoGrip going on for weeks) and the way they handle are not selling points, quite the contrary
But if one overlooks the physics and service problems, it is a great service offering a consistent way of accessing online simracing (if they keep the format as it is).