I found a great article on Warhammer Generals and their affects on morale on the Hendersonworks blog. They talk about the Black Orcs and Borgio from the Dogs of War. It's simple titled "Warhammer Generals" and as mentioned they are very important to your army.
Horde armies are especially subject to leadership issues if the general is not nearby or has not been killed during the battle. Plying an army like the Skaven with leadership scores ranging from 2-5 for core troops means you really could use a good general that can boost your mundane troops to a 6-8. The same could be said for goblins and night goblins. A basic goblin horde is testing leadership on a 5 or 6, so keeping them within 12" of your goblin warboss can raise the stakes to a leadership of 8.
For those who don't play Warhammer, making a leadership test for breaking from combat, panic or trying to attack a fear causing enemy, is rolling a pair of dice and equaling the leadership score or less. So if the goblin wolf riders are out on a flank away from your general, they'd need to roll 6 or less on two dice. However if they were within 12" of the general, they could test based on his inspiring presence of 8.
Other special rules to look out for are things like units being steadfast so no modifiers apply to their leadership tests and armies like the Lizardmen being cold blooded get to roll 3 dice and use the lowest two.
Also consider magic items for your army like the Standard of Discipline, this banner gives a +1 to your units leadership, but they cannot use the generals leadership so weigh your options carefully.
So keep your low leadership troops nearby your general and watch out for dark elf assassins and skaven trying to cast skitterleap!
Have a great battle,
The Old Crow