Anyone criticizing the Horde's withdrawal is not being objective. Essentially all the leaders made the same decision: they retreated when their position became impossible and staying would mean annihilation. This happened for the Horde moments before it happened for the Alliance, but it happened to both and all leaders responded similarly.
The demons in front of Gul'dan were a distraction, a holding action. The real mass was coming from our flank. If you actually played both sides, instead of just watching the cinematics, you know there were 10x, maybe 100x as many demons on our flank. My Tauren Bear was doing 124k dps with un-buffed thrash and swipe. I did nothing that dramatic in front of Gul'dan. They weren't in neat rows, they were just so many you couldn't count them, so many your screen became a green blur with red damage numbers swimming across it.
The Horde retreated with Thrall unconscious, Vol'jin mortally wounded, and a crushing weight of demons.
The Alliance saw that same mass of demons on their flank and ordered the retreat as well. Varian would have left with his troops to fight another day (like Baine and Sylvannas), but had to sacrifice himself (heroically!) when the Fel Reaver grabbed the ship.
It makes perfect sense to role play as if your character doesn't get this, if you're into role play, and especially if you're playing Alliance.
But any player who doesn't get it is not thinking it through.