If it hadn't been Rock, it would have just been Cena or Undertaker. Cena is ending up with the WWE Title around his waist soon enough regardless, and we knew we were getting Cena/Rock again as well, title or non-title. Might as well make their second encounter for the belt, in order to differ it from their first. Undertaker winning the belt would have been worse than the Rock, considering he would have either vacated it the next night (way to respect Punk's historical reign there, beating it but then saying "hmm, I don't really want the belt, here, you all can fight over it, I'm going back to retirement."), or lost it the following month at Extreme Rules or some shit. At least Rock winning it again is surreal, considering it's a sight that no one ever thought they'd see.
As far as Cena/Rock II... Can't say I'm looking forward to it more than most everyone else (on the internet, that is) but you can't really blame the WWE for pulling the trigger on a rematch that was so successful the first time around. How many huge matches in WWE history have not had at least one rematch? Very few. Everyone should have known this match was coming even before WM28, and that it was gonna be for the belt the second Rock announced the next night that he was returning to win the title again.
It also closes out the whole Mania trilogy of Cena and The Rock rather nicely - Rock costs Cena the belt at WM27, then beats Cena at WM28, but Cena returns the favor by both defeating The Rock and taking the WWE Title from him in the process (the same belt Rock cost him two years prior) at WM29. (It also puts dent in the 'Superman' armor that Cena has been accused of having, by showing he CAN lose a big match that he is sure he will win, and has to come extra hard in order to defeat said opponent.) In the process, Cena gets to go down in history as the last ever guy who beats The Rock for the WWE Title, just like Punk also gets the honor being the last ever guy the Rock ever defeated for the belt, which is an 'accomplishment' in itself.
Punk fans should really rejoice - they never thought he'd not only get such a long reign, but as someone on another forum pointed out, if you said 5 years ago that Rock would return to the WWE and Punk would be only one of two different one-on-one opponents that Rock would face, everyone would have likely laughed and laughed hard. But now it's become a reality. And now Punk's even likely to get a Wrestlemania match with the Undertaker, which is an honor as well at this point, considering Taker only faces established main eventers (Batista, Edge, HBK and HHH twice.)