Mage armor only affects meditation and not focus, because you can actively meditate and (as far as I know) you cannot actively use the focus skill.

Mage armor affects paladins the same as mages really, it just depends on how much you use active meditation. If your paladin's mana doesn't often get too low then mage armor isn't that important.

I don't bother with mage armor for my paladin because they just don't use the paladin spells enough to run their mana down, but if you run out of mana often then mage armor could really help.

Basically, mage armor only affects you when you actively meditate (because you can't meditate with normal armor). It least, that's how I've always understood it.

Hope that helps clear it up a little