early on you still are going to wear warrior gear to get to the 490 cap, pure paladin gear will not gear you there, it is also itemized to have significantly less health than the warrior non-set pieces. Such as the Panzar’Thar Breastplate. Or pre-raid you will be wearing Felsteel pi...
The way paladins get around this early on, is to simply wear warrior gear with no paladin stats on it, and have lower spell power. The bosses in raids tend to completely bypass argent defender(hence why it was changed in Wrath to allow you to not die on a cool down if your health ...