I play healer 90% of the time and as such I love when a, well played, paladin is in the group. Tanking or not, a paladin bring a lot of ability that greatly increases a party's survivability.
Just a few things I've seen a great paladin do, when not tanking.
-Time his rampart with the warrior;s rampart making rampart up 100% of the fight.
-Use enmity WS with collusion, granting extra enmity to the warrior.
-Throw cover on the tank when the tank had low hp, get a block, use spirits, and help heal the tank.
-Covering melee so he can start is front combo line,
-With single WHM run the tank died. The paladin took threat, used Hallowed ground, tanked while raising the dead tank. Then took over tanking.
-Take over healing when the WHM died. No really! Everyone gets better mana regen when in passive mode. He healed until the bard could get the WHM back up.
A well played paladin is fantastic and can replace the need for a second WHM. I love running with paladins, sadly few groups let them. Good paladins stop wipes from happening and have the tools to do it.