My experience is that the best talks have useless slidedecks. That's because the decks support the talk and do not provide a foundation for the talk. Less good talks often use the deck as a documentation platform; in that case decks become much more useful for reading offline. The exception is when decks are commented heavily for people reading it offline. But I don't see this happening often. Usually, comments are used for the speaker to have some information ready when needed during the talk.