A fully improved (3/3) Cleave will do Weapon Damage + 110 to two targets at Rank 5, instead of Weapon Damage + 50, which is a significant improvement for just three points spent.
Cleave, Whirlwind and Sweeping Strikes are the only meaty AOE attacks the warrior has, so any way to improve either is worth looking into, and Cleave's stance independence (usable in all three) makes it especially versatile for coping with adds, or just pulling two at a time. Helpful for tanks and soloists alike.
Alternatives at this tree tier are Piercing Howl, Blood Craze and Commanding Presence, all very useful talents, but Improved Cleave only goes to three ranks anyway, leaving room for Piercing Howl and one point of something else. This tier is so useful however, that it may be worth spending more than just the five points needed to progress further down.