I guess initial idea for rewards for voting was introduced because of two reasons:

• being paid for something what you are doing on facebook for free - in theory that sounds like a good PR phrase
• there was a big problem with lack of interest in voting for worker proposals in BitShares

But Steem is different than Bitshares and IMO, it will never have problems like bitshares have... mostly because decision that those money for post will be spent, that decision was already made. You can only influence, how those funds will be distributed.

In Bitshares everyone wonder whether it is good idea to spent money in the first place.