Keeping the cost of entry free so all can participate vs. requiring some skin in the game. I can see the merits of both.
No fee to enter allows everyone in, spreading Steem and Steemit faster and to all parts of the world. Skin in the game might make people more thoughtful about what they post, but inevitably, as you say, it will go up and eventually make it too cost prohibitive, if not go all the way to the point where only those with lots of money can pay to play.
It's a dilemma, and it's really at the heart of not only the spam/plagiarism situation, but the debate over the 'proper distribution' of rewards. Having free entry makes the reward pool seem like monopoly money to a lot of people—even to those who should know better—and it's hard not to think of it like that when you haven't done something to acquire it.
I wish I had the answers. There's so much I feel I need to know. But at risk of overstating the obvious, this issue is only going to get worse when Velocity arrives.