Shows like Rocky, Bullets, Mermaid, etc performed b/c they missed on the big prize but producers still wanted to take advantage of the biggest advertising night of the year.
Try telling that to Michael Riedel who just keeps assuming that CBS wants to put out more entertainment.
If they were going to do something for Mamma Mia I think they would be best served by incorporating that into the opening number. It seems you are more likely to get people to tune in and stick with an awards show if you give them something that will excite them at the start, rather than dragging out the promise of "coming up soon." And yes, I do think a number from Mamma Mia will pull in more of the casual viewers/theatre-going public than an something unknown.
While you do have a good point, I think a much more exciting opening number for this year's Tony Awards would be the cast of On the Town performing New York, New York.