The thing is, implement powers were meant to represent things that might not actually involve explicit attacks, or were not really 'weapon-like' (say something like sleep or some kind of charm or something). Even Divine characters like paladins do this kind of stuff sometimes, so it seemed flavorful and logical to give them implements for it (and certainly the Chaladin finds them fairly thematic). I can see a sword working for a paladin as a substitute, as you outline. However it is ODD that they wouldn't be able to use a holy symbol, otherwise you could just make all their attacks have a weapon keyword (but with whatever ranges you wished, a bit outside the norm but 4e can express that).
However it doesn't work so well for the cleric. The real problem IMHO is the mechanical division of powers between 'weapons' and 'implements', maybe they should all be mechanically equivalent. Instead of having to enter the swamp of 'weaplements' and its attendant feat muddling suckiness, maybe the whole idea should be rethought! This is where I'm stuck right now, how to express the same sort of idea, that you need 'tools' with certain characteristics to make your powers work, but that they may not in fact need to be specific things.
Actually I've been thinking about just making them all equivalent, that is getting rid of the concepts "weapon" and "implement" as rules categories, and just specify what works with a given power.
