#1 He would only be used to put Reigns over
We have already seen it happen; most of the heels in WWE are used to put Roman Reigns over. We have seen this happen with Braun Strowman, Seth Rollins, Kevin Owens and Chris Jericho. Jinder Mahal was personally brought back from SmackDown to Raw for that only. The same fate might await Dean Ambrose if he turns on his current buddies.
He would be used to put over Roman Reigns as we all know Dean can simply make the crowd boo him (turning on The Shield would simple make the WWE universe hate him). The fans would simply not like 'The Lunatic Fringe' to just put over Roman Reigns.
A female WWE star said her life is in danger. Details HERE
A person as talented as Dean Ambrose simply deserves better than that. He should either stay a babyface and turn heel when he goes to SmackDown Live if that's the case.