I think the big thing here is money. Yes the B12 wants their teams in the playoff, but was the conference hurt financially? If your reasoning for adding 2 teams is to get to the playoff, what is the financial advantage of that? I believe the conference stood to make $2 million more if they make the playoff. It is very likely adding two teams would dilute the payout to the current 10 member schools by $2 million each. That math isn't very hard to figure out. The B12 will have plenty of years it will make the playoff. If this is a trend vs a one year happening then we can argue that adding 2 teams at dilution might be worth it. But to kneejerk that we need 12 as the detriment of $20+ million really doesn't make sense.
This article from Bring on the Cats was posted in one of the other expansion threads. It makes a good financial case for expanding to 12. Basically, the conference would be just as well off, if not better off by adding almost any two teams.
http://www.bringonthecats.com/2014/12/7/7350401/big-12-expansion-why-it-needs-to-happen-and-how
It's not just the money that comes from the playoff. It's also lost money from having TCU and/or Baylor taking up bowl spots from other schools, along with fewer losses on average among all teams due to conference games (more bowl eligible teams).