Handle Animal is a skill reflecting your ability to calm and train animals. It uses the Charisma stat as a modifier
What can you do with Handle Animal?
This skill allows you to both train and prompt animals to perform tricks. You can handle an animal, causing it to perform a trick it knows, or push an animal to perform a trick it doesn't know. You can teach an animal a specific trick, or train an animal for a general purpose giving it a suite of skills or even domesticate a wild animal. You can teach an animal to attack, come, defend, flee (down), fetch, guard, heel, perform, seek, stay, track and work. An animal can be trained for combat, fighting, guarding, heavy labor, hunting, performing and ride.
Which of these have ground for use?
This seems more like a profession then an individual skill. I believe it's been moved over into the territory of an individual skill because it has applications in combat. Most of the skills and their subsystem seem reasonably applicable as a system for training and using animals in a campaign.
This skill really comes down to frequency of use. Do you use or command animals enough to be useful? In every campaign I've participated in, this skill was never used for animal companions and there were rarely other animals in the game besides that.
The general idea is that a character gives up his move action to convince an animal to do a trick, and can spend his whole round trying to get the animal to do a trick it doesn't know. Druids and Rangers get an advantage in allowing tricks as free actions and the ability to push an animal as a move action.
I've played Druids before, many times, and have never recalled being asked to roll a handle animal check. The animal was just treated as an adjunct to the player, performing whatever task I as the player decided. How many DM's just hand-waved the skill and let the Druid or Ranger use the animal as a secondary weaker PC? All of them.
Is this not how most tables ran druids and rangers?
What is it we gain by having this skill?
A method for interacting with and training animals.
What do we lose?
I am not certain that this is a particularly effective way to demonstrate the vaunted 'character differentiation', but the d20 roll and the use of this skill works just fine as a model for training and commanding animals. Even though the roll is a pass/fail roll, the fact that you're influencing animals allows you to treat failures as partial successes, leading to interesting game-play.
Conclusions & Suggestions:
I think this system for animal control is fine. If actually used, it could mitigate some of the 'I have an animal companion so I get two turns on my turn' issues with Druids and Rangers.
Alternately, being that I've never had it used when playing a Druid, Ranger or Mounted Knight, I could also just eliminate the skill entirely.