If you play a Paladin, you get access to all skills that Gladiator gets on leveling (Including the level 50 AoE attack you want), as well as 5 unique skills that only Paladin can get (Including a Defensive stance that reduces all damage taken by 20% and boosts aggro generation) in return for losing out 5 cross class skills. So the major disadvantages are 1. You don't get any Paladin Skills, 2. You don't get access to the massive stat boosts the Job provides, 3. 90% of all parties will resent you for not having the Paladin skills and stats, and 4, you can't get the best gear for your class unless you have Paladin.