For those of you that don't understand how holding a button is better than toggling it, this is my reason:
In real life, when you aim or crouch, standing up straight to run and/or lowering your iron sights is done without thinking. It is a natural, instinctual action. When you aim or crouch, you have to "hold" that position. Crouching takes more effort than standing (at least for me since I'm tall), and aiming takes more effort than not aiming.
Now, I set my controls to hold the button for aim/iron sights and crouch. Let's say I am crouched and have your iron sights up and scanning for targets...OH NOES, A GRENADE!!! GET THE F OUT OF THERE!!! Instantly, I am able to let go these buttons without a thought and sprint out of there. Those that have to toggle their iron sights and crouch have two extra actions to complete/think about before running away. To me, this is slower and less efficient. And that's not the only situation where this is applicable. Taking enemy fire is another instance where my control scheme comes in handy. I believe that my setup of holding down a button for certain actions is more fluid, faster, and closer to real life than toggling those buttons.
Maybe with practice, toggling iron sights and crouch can be instinctual, but I still think my way is faster and will always be more intuitive to me. Holding a button for iron sights doesn't seem to hinder my aim, and I've used both toggle (Killing Floor) and non-toggle/hold (RO 1 & 2) before.