Also, one more argument has come to my attention:

Currently the potential weakness of Steem's economic foundation is that the user does not have a clear incentive to buy the token - she can still enjoy the platform as a passive reader without paying anything. This cannot be changed as people are used to the fact that internet content is perceived as free of charge.

Whereas with prediction markets being integral part of the platform the situation becomes quite different. It's obvious that in order to bet you need to put the money on the table. And here comes demand for the token...