Shock value. No one really saw Seth being the one to turn on the Shield. He was always the peace keeper in the group. Breaking a group like the Shield, at the time, needed to be shocking and make people care. Really it should've been Roman to turn heel on them, but even that would've been predictable.
Can't lie that it worked out. Rollins was a great heel IMO and Ambrose was one of the most over guys in the company for a year and a half or so. His title winning moment arguably made it worth it. Plus, that Rollins/Ambrose feud after the split was one of the best feuds of the past several years. At the moment things have gotten stale, but they could always decide to turn him. Deciding to go babyface 3 years ago doesn't mean they are locked in to keeping him that way.