Razorace: the problem is that when you ledge grab, there's a 2 second interval before you can get your character to react to either let go or climb up. During these crucial 2 seconds, it could mean being shot and getting killed instantly
Also, it is more realistic to have to hold jump a second time in order to ledge grab. For example, if you're falling at 50kmph, there's no way you can grab a ledge without tearing off your fingers. You'd have to slow down first, then grab it.