Because I'm bored:
Keep in mind that promotion bonuses are based on the class bases. Mages have typically some of the worst base Defense stats, while mounted units with medium armor like the Dark Knight often have good defense bases.
For reference, Mages have 2 base Defense in FE12. Paladin, a class I assume has a similar armor type to the Dark Knight in this game, has 8 base Defense. So promoting a Mage and reclassing him to Paladin would effectively give the unit 6 Defense.
Then again, the Magic, Speed and Resistance boosts are either low or completely absent. This might not be as awesome as you'd think.
Mage --> Paladin (FE12)
Mage --> Dark Knight (FE13)
Mage --> Paladin in FE12 also gains +6 HP; I'm guessing Mage --> Dark Knight has a similar gain, but I can't see the base HP stats to make sure. Of course, Cleric --> Battle Cleric gains +8 HP, and Dark Knight looks even bulkier.
Edited by Othin, 05 April 2012 - 01:17 PM.