after leveling Paladin ... Dark Knight just feels really squishy

[TouchandFeel]

Pretty sure this was the general principle of where you were going with it, correct me if I'm wrong.

Pretty much.

I didn't go much into it previously but you also need to account for the fact that Salted Earth is a two-click free roaming/free target cast which, while ogcd, still can take longer to get out than many realize.
Every time you use it you need to click once to activate it, check for the location of the ground marker, adjust the ground marker as necessary and then click again to place it. Even if you are very practiced in doing this it can easily lead to clipping your other attacks and abilities during a period of high activity such as the first few seconds of stopping to fight in a big pull where you are trying to establish your group aggro lead and use your defensives since that is the period of highest incoming damage.

In addition to the potential of clipping or delaying abilities, the couple ticks missing a few mobs at the very beginning is often, if not always, made up for by increasing the amount of time Salted Earth is affecting the mobs before they start getting killed off.
You are going to lose ticks of Salted Earth on some mobs, whether at the beginning or later as the mobs die while Salted is still running. Prepping the cast and getting it out at the beginning just shifts where that happens and frees you up to utilize your time and attention to the myriad of other abilities you want to get out as soon as possible.