An extra army certainly won't hurt. But in a long game, creatures that join you won't be very useful unless they are from your faction. Of course, you can hope to encounter neutral orcs (which is likely to happen on a large map), but even with diplomacy, they may not join or may ask for a hefty sum of gold, which you may not always want to give. After all, there are more useful skills in leadership. Just the aura of speed and the battle frenzy are worth it. ;)
It's also worth leveling up to "Toughness," which will also help in battles against the living.