Well, it wouldn't be just for the sake of him being there. Granted he has the guaranteed contract, but it's not mandatory that he cashes in before WM. He could, theoretically, wrestle the champ at WM while still Mr. MITB, which would also add another layer of drama to the whole thing being that the could just cash in afterwards if he lost, or get himself DQed to take advantage of the briefcase or whatever. So there would be a logical reason. Plus, in kayfabe I'm sure Rollins would enjoy being able to say he won the RR match.
Yes, Rollins being Mr. MITB has him a guaranteed title match at any time and any place. I see where you're coming from.
But, the odds of him participating in the Rumble and actually winning it amount to zero... Hence why I said not to have him there only to get fed to Reigns.
Unless they're planning to trigger Ambrose/Rollins feud again at the Rumble, then I'd be okay with it.