Speculative reasons: the guess that some may have made by now that the value of Steem coins/power/mist whatever the unit is called might grow just like Bitcoin and that investing a grand into Steem now regardless of any curation or posting rewards might return a nice 10 grand or another very profitable number. I think whether or not curation rewards become discarded, and whether or not interest even becomes discarded the mere fact that 10 Steem coins may become worth 10 BTC sometime in the future is a very integral part of providing incentive to ensure the Steem community grows healthy