I don't either. You also need to create tension in the main event, and having Roman there does that.
Exactly. The only way I can see Ambrose getting into that main event is if there are shenanigans at Fast Lane that makes it a triple threat. You have the tension of Reigns vs HHH and you also have the tension of Reigns vs Ambrose. Reigns could feel that Ambrose has his back against HHH but in fact he has his own agenda to win the title. Or Reigns could even say that Ambrose will just get in the way of him beating HHH.
That way you could have a short feud between Reigns and Ambrose over the title at Extreme Rules and Payback. Rollins, who should have returned by then, will demand that he gets title matches to reclaim the belt he never lost so MITB and Battleground will be Rollins vs the champion. Then in the build to Summerslam all three of them start butting heads and it leads to Reigns/Rollins/Ambrose at Summerslam for the title.
Yeah I know the Shield triple threat is a Mania-level match but could you see WWE waiting a year to do that? Unless they do a slow build towards it. Plus if they're pushing Summerslam as a bigger event than usual, that kind of match can help out.