Many horses that I've worked with were not so well behaved when just tied to a post. They'd tug on my quick-release knot until they'd get enough length to reach that oh-so tasty blade of grass. Eventually they'd untie themselves and be munching away. Okay, so maybe that was an issue with training (or my knot tying skillz).
In cross ties, the horse clearly learns his boundaries because the ties always stay at the same length. I guess for me, it's just easier. I also find it easier to groom being in cross ties because you can go around the front of the horse without having to duck under a hitching post.
My horse was uneasy about using cross ties at first, so I just tied him to one side. If he freaked out, he had enough range of motion to spook, swing his butt around, etc and then chill out. Now, he knows exactly where to stand in the isleway to be comfortably tied in the cross ties. He has enough length to turn his head and look behind him, or drop his nose and relax after a tough workout!
If you ever plan to sell your girls, I would recommend getting them used to cross ties. Heck, it's just another positive thing you can add to their sale listing! "Stands quietly tied or in cross ties."