Well, if a mage has a AMF "spell", a problem occurs that the mage will have to overcome its own AMF in order to keep the spell running. This will make it an overly inefficient way of protecting oneself from spells.
Theoritically, a mage will only be able to keep an AMF field going that's half the mage's power, while using the other half of the mage's power to overcome the field itself. It would seem to be far more efficient to use barriers and shields as necessary as that would allow oneself to use additional magic.
One the other hand, AMF fields can be useful if activated by an external device. For this to be useful, the user needs to be proficient at non-magical combat. An advantage is that the user does not need to be a mage.
This is just starting to sound a lot like the game Saga Frontier 2.
…and I got hit by the redundant-acronym syndrome