Let's put it this way, there is nothing prohibiting you from controlling a 3 phase motor with a 2 pole contactor. The only requirements are that you protect all three phases from short circuits and ground faults, and that the motor has running thermal overload protection. If the thermal protection is inside of the motor already, you see this done quite often on cheapskate HVAC equipment vendors. That extra $3 for the 3rd pole is too rich for them I guess.
But if you are using an external (to the motor) overload relay, there is a requirement that the OL relay protect all 3 phases. So for motor starters, which are a convenient way to accomplish both the control and protection, it's easier to just get your 3 pole OL relay attached to a 3 pole contactor. Nobody sells a 2 pole contractor with a 3 pole OL relay. You could, I suppose, but a 2P contactor and get a separate 3P OL relay, then mount and wire them together yourself, but the $3 you saved on the contactor cost you 10 or more minutes of your labor. You then worked for about 50 cents/hour...
Why is it not good? No good technical reason really, but it tells the user that you built something as cheaply as humany possible. In reality though, even though it is technically illegal and a REALLY bad practice to rely upon a contractor to isolate a load from the supply, it happens nonetheless in the hubbub of field repairs, often done in a hurry. Leaving one pole live when the machine appears to be off is just asking for trouble.