The problem is that people assume that football happens in a vacuum. When a traditional power is down, they assume that means the league is down. There's just one thing about that assumption: it's wrong. When one team falters, another team (or multiple teams) rises up.
Look at the former Big 12 North. For the early part of the Big 12, Nebraska, Colorado and Kansas State were the elite teams. When Nebraska fell back to the pack a bit, and Colorado and Kansas State fell off, Missouri and Kansas emerged as the top teams in the league. It's not always a straight one-for-one replacement, but the point is valid: when someone goes down, someone else will go up.
And that's why the assumption that the Big 12 is Oklahoma and Texas, and everybody else, is so inherently flawed. True, the one thing about traditional powers is that they typically field stronger teams on a more consistent basis. But that doesn't mean that the other teams in the league won't field awfully good teams from time to time.
The issue is that those teams haven't always come from the same source. In the Big 12, it seems like a different team emerges every year out of the blue. In 2007, Kansas and Missouri each won 12 games. In 2008, Texas Tech tied for the South crown with Texas and Oklahoma. Last year, Oklahoma State won the league, and around and around we go. In fact, I would argue that the ability for all of those teams to emerge quickly is what makes the Big 12 such a strong league.
So maybe there isn't a solid No. 3 (or maybe Oklahoma State can work on becoming that team). But try this stat on for size: in the past five years, every Big 12 team has won at least 10 games in a season except for one (Iowa State). And the Cyclones haven't exactly been easy fodder themselves, going to a bowl in two of the last three seasons.
That's why, whenever somebody mentions the Big 12 being weak, or Texas and Oklahoma having an easy march to the BCS National Title Game (or playoffs), I think you almost have to laugh at them.
Because while there are weak spots in any league on a year-to-year basis, the Big 12 has shown a unique ability to produce winning programs at every school in the league. And that's not a weakness. That's a strength.