Imagine if people couldn't lean in real-life. Instead of a soldier leaning around a corner, he'd have to actually step out of cover and expose half his body. Ridiculous? Indeed.
The only thing that's ridiculous is you comparing the work of a solider to that of a video game. There's a whole shit-tonne of stuff you can do in real life that you can't do in a computer game but that doesn't mean it isn't immersive. I mean, free head movement for one - the way the viewpoint is fixed on where you're aiming. Surely that's more important than lean? And yet that's not a common feature (ArmAII has it but that's about it).
I don't really see how finite control over how much you lean is really necessary.
You were talking about immersion earlier and yet you don't have a problem with the degree of lean always being exactly the same? "Here I am standing, oh wait now I'm leaning at 45degrees". Far Cry 2 had HUGE immersion because of the embodiment of the player in the character and yet you're moaning about lean, which has a much lesser impact upon immersion. For all it's problems I think that Far Cry 2 was one of the most progressive games in recent history. If they work on addressing the main issues in the sequel then the game could be truly special.