I think as "real" race car drivers, expecting a sim, ANY sim to be a realistic training tool is.. Well, unrealistic.
I drove on iRacing for hours upon hours before I bought my vee and I really feel like it helped me tremendously in getting up to speed much faster in the real world. But expecting it to match your real world driving, braking points, shift points, etc will definitely disappoint you.
Firstly, speaking for myself, I'm a lot braver in the sim than in real life. I'm not babying my FV around the track, but I do leave some self preservation margins here and there. On iRacing, every lap is an 11/10ths qualifying lap. I have lots of spins and make silly mistakes on iRacing that I wouldn't do in real life, but that's not really their fault! There's a limit to my concentration and focus, and to how long I can drive on the absolute edge. In real life, I try to save that for when it really counts, in the game I push myself every single lap until I can barely make two corners in a row.
All physics and feedback problems aside, it's a lot harder to drive around in a sim than a real car - it's harder to focus and easier to get distracted, there's no seat of the pants feel, no tactile feedback (well.. not with my budget, anyway). So, personally, the biggest value in racing sims is exactly how hard they are. Most of my real life club races are 20-30 mins long and I have plenty to keep me busy and nothing to distract me. On iRacing, I've done hour long races with my phone ringing on the table, the dog begging me for food, the girlfriend coming home from work, while trying to see which way the track goes on a single 20" monitor. You're damn right it's easy to spin a car on iRacing! Try driving one around in really life, while keeping up with the pace of the leaders, drinking coffee and eating crackers!
Anyway - iRacing has helped my focus a LOT. I'm a very easily distracted person - I crashed on my second real life race cause I was waving at the flag station and missed my braking point. I credit iRacing for changing that for me and keeping me sharp and focused for a whole session.
PS: The only reason I use iRacing over the other sims is I don't need to mess around with settings or plugins or mods or whatever. Tried rFactor for a while, the stock setup was horrible. Found all the links for the realfeel mods, and the car mods, and the better tracks.. Meh, I'd rather pay the 7 bucks a month, click "GO" and be driving! iRacing worked perfectly with both my old Momo wheel and the newer Logitech, when I decided to build my load cell pedals, I plugged them in and they worked right away. The physics aren't perfect, but they're pretty damn good for what it is - a tool, not a replacement!