WEll, there's also a couple limitations: Magicians can tell there's a spell, since it's not a mana-based illusion, you get the image of that big fire elemental, but not the astral signature. If it's flat-out unbelievable, people will get a bonus on the disbelieving bit.
That being said, a real douche of a magician CAN and should screw with people on that. Make the illusion of that huge, nasty elemental follow your spirit around. Or, make your big, nasty spirit look like a smaller one. Make that gap the person needs to jump seem just a bit shorter-or longer. It's a powerful illusion, yes. It's also high-drain, and it can, if used right, really ruin somebody's day, much like any spell. Take a look at the Energy Aura spell. It needs to be maintained, on the other hand, slap that puppy on your Melee specialist, and watch him tear into everything around him. All spells have limitations, and Trid Phantasm is also a spell which is most widely used in the entertainment industry because cameras can catch it.