most people run 2500 or less crit resist, that's about 35% crit resist. This means that even with base chd modifier you're doing 15% increased damage on crits on targets with 2500, with 50% crit chance that's a 7.5% expected damage increase. To get the same increase from penetration you'd need 4,950 penetration. With a CHD modifier of 1.75 you'll see a 40% increase in damage on crits for the same target, that's a 10% damage increase at 50% crit and requires 6600 penetration to match it.

As you can see, once you've got adequate base damage, crit damage is a very efficient way to increase your burst, particularly since elfborn/precise strikes also increase your crit healing whereas investing into spell erosion/penetration will only increase your damage. Now, due to diminishing returns you want both, I'm not suggesting ignoring penetration, just noting that crit chance and damage are both very valuable as well.