ok paladins are granted powers by their deity
if the deity is a thunder god it would fit
but this would only be some spells, not all of a druid
i think paladin is not a good expample to talk about multiclassing
a paladin is a hybrid like a reaver
if i think about a dark paladin then its a reaver! but like said before its only my point of view and shall not be the ultimate answer! so please no anger against me!
if we would have 200 lvls then no one would multiclass like hell anymore! all would gain experience in their main class and do some lvls to enhance. but at lvl 100 there is no further way in this school!