Ok...
I really don't know how this motivated them any more, but it can't be any worse than the killing and beheading they are doing to the troops and prisoners now. A prisoner can only get so dead.
It's NOT worse, by any stretch than Zaqarwi gutting people on video BUT let's turn this around to make the point of how it affects the battlefield:
If you are Achmed and you're fighting US Marines day in and day out and getting your hat handed to you, time after time and you're starting to think this is starting to not be worth it and your comrades are starting to go home, to leave fights at first contact, to not show up for tactical meetings or ambushes and then, suddenly, your leaders are jumping up and down like organ grinder monkeys;
"Look!" they say, "Look what will happen to us ALL if we give up the fight, if we lose, then US will let it's women chain you like animals!"
My point is that it helps the moral of an enemy who might otherwise set one less IED or might have a few less hands in a firefight. There might be 5 or 10 or 100 Gomers who stay home in Saudi or Syria instead of running off to fight the Infidel if there's no Abu Gariab.
Our kid worked intel and he knows what happens on the battlefield in relation to X event. He can't say jack in detail but if his opinion is that it does not help then that is different and worse for our grunts than an opinion that it doesn't matter.