So, this warcry doesn't affect morale:

It affects the mind, and the undead are immune to mind-affecting effects.

Added 2 minutes later

I completely agree - why would they even be given the ability to learn it then??

To give players the opportunity to develop their heroes in unconventional ways. Maybe you decide to play with summoned elementals, and you really need to cast some of the undead's skills on them, or on neutral creatures that have joined you. In general, it's for the players' strategic development.

Added 3 minutes later

It's like it appeals to heroism, which the undead completely lack.

That made me smile. Players always come up with more ideas than the developers, and in Heroes 6, the undead are actually heroically fighting against demons and angels.

UPD: Does anyone know what this egg is, which pretends to be an artifact, but it's unclear what properties it has. The description says it's a mysterious egg >_<

After N battles, when it's in the inventory slot, a goose will hatch from it, also a golden one