I love the ledge grab! It's one of my favorite features. I've been a fan of the ledge grab since Tomb Raider.
Holding a button could work (I would suggest the Crouch or Walk button for that). However, I've always found the grab to usually be quite intuitive as it is. I dare say I like it better when I automatically grab a ledge, especially since it can be a last-ditch "save" you never expected to have as you get kicked off a ledge. Very dramatic.
As far as the real-life difficulty of pulling up on a ledge... my experience is that it's easier by far than doing pullups on a bar. Having a surface to balance your body against helps a ton, even if for some reason you can't use your feet.