#3 It would make 2018 significantly more interesting
For the longest time, 2018 was set in stone, at least as far as the major storyline on Raw was concerned. More or less, it went like this:
"Roman Reigns is going to get a title shot against Brock Lesnar at WrestleMania, either by winning the Royal Rumble or an Elimination Chamber match. He will then slay the Beast and have a Universal Championship run that could easily last to the end of the year or longer, defeating all challengers."
I don't think I'm alone in saying that doesn't strike me as being particularly exciting.
It's inescapable that Roman Reigns is going to WrestleMania, but the presence of The Shield would be a spice rack just waiting to make that bland meal tastier. How do Dean and Seth factor into the buildup? Do they try to help Roman?
Does Roman want their help? Does Brock Lesnar try to mess with Roman's head by laying out his teammates? Or maybe Paul Heyman gets into Roman's head by challenging his manhood, chastising him for relying on his buddies and challenging him not to? Is there any jealousy present that threatens the unit's cohesion?
And after WrestleMania, with The Shield holding the Tag Team and Universal titles, who rises to challenge their dominance?