Yea it would be much different, in that it would be the same as last year,which would be boring. Especially with so much time between The Rumble and Mania, and Reigns not being there half the time. Rock adds a different angle to it which gives everyone involved something new to work with. Cody has been great but his focus has been on Rock. Reigns is now interesting because of the power dynamic with him and The Rock.
Without this they have what, 2 months of Cody having to talk to himself giving promos and then Reigns shows up every 2-3 weeks so they can stare each other down? The same rubbish they've been doing with Reigns for god knows how long.
Last year they had Sami to make it interesting, this year they have The Rock. I don't think it's a coincidence that all this stuff tends to happen on Smackdown, and I generally find it an entertaining show, while Raw I rarely manage to make it all the way through.