The match should've happened if Lesnar went undefeated and destroyed all the guys he fought (Cena, HHH, Punk, Orton, Sheamus, and/or whoever else). Rock/Lesnar might be better for short term business, but long term it would be better for Rock or Lesnar to put over the next top face of WWE... whether it be CM Punk or Sheamus (though Sheamus isn't ready for that match yet, but maybe could be by the time Mania rolls around). Whoever doesn't can face Taker for the streak. Taker/Lesnar would be better for a storyline perspective, but Taker/Rock would be bigger for business for mainstream. Both would be huge for wrestling fans.
However, even not taking into account this report, Taker is clearly facing Lesnar if between the two of them. Rock's already basically declared he's going after the WWE Title. Taker isn't winning the WWE Title ever again. That means Rock is either facing Cena again and Cena wins the title at some point, or Punk is champion by the time Mania rolls around.
Personally though, with Lesnar losing to Cena, I'd much rather see Taker/Rock. I want Taker/Cena for WM30 and Taker's final match and I think that is what will happen. Plus with my brother being a massive Rock mark and me being an equally massive Taker mark... well, it'd be like old times.