Right now it seems pretty evident that Didi is going to be a MLB player. He has all the tools and he's developing them. I think the only question is: will he be a good (and cheap) middle infield bench guy or will he be good enough to be a starter?
If Didi continues progressing on his current track and is able to carry that over to the majors, then he's a starter - that defense/hitting package at SS is too good to make him a part-time player. If he develops to that point (and I think he probably will), then I think the Reds have to trade him or Cozart (a la Larkin vs. Stillwell).
You generally platoon players who have been given a chance but have shown they have a glaring defect (i.e., they've proven to be pretty bad against either LH or RH pitching). To just platoon two players because one hits from the left side and the other hits from the right side stunts the growth of both. And, in my opinion, is a huge waste of a valuable asset. Remember, the Reds got Danny Jackson for Stillwell (Ted Power and another player from the Royals were in that deal too, but Jackson and Stillwell were the key pieces).