Yeah it's important to remember that a holy symbol is not a material component, it's essentially a weapon and is expected to have magical bonuses with time just like a sword does.
Eventually a paladin might be able to have a +2 shield and have this count as his +2 holy symbol, but this is certainly not something that should be assumed. And even then I'd expect a paladin's weapon would become his symbol rather then his shield (because your main hand is I believe supposed to be the slot that boosts your powers).
Eventually a paladin might be able to have a +2 shield and have this count as his +2 holy symbol, but this is certainly not something that should be assumed. And even then I'd expect a paladin's weapon would become his symbol rather then his shield (because your main hand is I believe supposed to be the slot that boosts your powers).