Dog/Cat form is good for tanking (At 120 ninjitsu you have 40 HPR!!!) throw in some mirror images and it's amazing what you can tank.
All of the fast movement forms (lama/ostard and up) allow you to stealth more quickly. (I prefer Kirin form for the fast stamina regen.)
Even though you have a timer before you can change forms after you dismount, ianimal forms can still give you an advantage in PvP. Dismount someone (running foot fight ensues), after the dismount timer expires change into wolf form (you can do this on the run much faster than they can mount an ethy)... Voila! You now have a movement advatage, +20 hps, and +20% HCI.