I feel like morghasts actually manage to find a niche in Ossiarchs that just isn't as useful in legions.
Like people have previously mentioned, they are the only Fly you get outside of "move like fly" on stalkers, nagash, and arkhan, and they give themselves the 6+++ thanks to the hekatos keyword.
In petrifex, they can be damn hard to kill and can soup up their output with extra rend if they need it.
In stalliarch harbingers become even more of an early game threat thanks to run and 3d6" charge, add in that +3" movement and you've got an average melee threat range around 24"-25". Throw in the battalion with stalkers in and your harbingers get to add an extra 6" to that on turn 1.
Ivory host lets them hit more accurately without requiring much in the way of support, but you do sacrifice some survivability for it.
So while they didn't get any actual changes to their warscroll, and stalkers and immortis are damn good, I'd say morghasts are more worth their points in this book and are different enough from our four-armed friends to be a viable option.