Another thing, and I don't fully understand why, is that a stripping basket is absolutely required. When fishing freshwater, I just let the line float downstream and the cast picks it up off the water just fine. Not the case with saltwater. Maybe the salt adds surface tension or something, but saltwater grabs your line and won't let go. A stripping basket effectively added 10-15' to all my casts. I made a home-made jobby out of a plastic tub, tie wraps and a bungy cord.
Water, whether salt or fresh, adds surface tension to the line. In the salt, you also contend with moving tide and weed, too; the LMD gets your line out of the water. The minimal impact you're noticing to casting when in freshwater is probably because you're fishing with floating line; salt, you're probably using intermediate. You can figure out the different impacts to your casting.
To me, if you're trying to maximize your casting efficiency and distance, an LMD is an absolute must when beach fishing.