The one cool thing about turning both Reigns and Ambrose heel and having them become Seth's new muscle (other than it being a genuinely shocking and unexpected moment) is that we'd get to finally see 'The Shield' target Brock Lesnar when he challenged Seth Rollins for the WWE Championship at Summerslam.
To date, The Shield has targeted and decimated nearly everyone who's relevant at some point (John Cena, HHH, Undertaker, Randy Orton, Batista, Sheamus, Big Show, Daniel Bryan, CM Punk, Ryback, Kane... hell, they even attacked The Rock) except for Brock Lesnar. He's the biggest star to date who's been spared the wrath of their vicious gang attacks, and so when he and Rollins clashed at Summerslam, it would be cool to see the three of them gang up on Brock and triple-powerbomb him through a table or something (or hell, maybe put him through both tables and then triple-powerbomb him once in the ring, too... a trifecta of their triple-powerbomb signature move, on top of hitting all of their individual finishers on him as well), which will lead to them standing over a fallen Brock with their fists pumped together and spending the next subsequent several weeks talking about how they've now conquered The Beast and stuff.
Beyond that, there's pretty much zero good to come out of turning Reigns and Ambrose heel and having them become Rollins' whipping boys, nor is that one awesome moment I mentioned above worth it.