Let me throw a hypothetical out there for you all to see what you think.
Let's say ND joins for non-football and ends up playing somewhere between 4-6 big 12 teams in football every year. You know the teams they end up playing annually will be big names...they won't be playing ISU (or if they do it will be rare).
Here's the question. The B12 Tier 1 deal with ABC is due about the same time as the ND deal with NBC. Let's assume Notre Dame wants to keep NBC as a TV partner. Who do you think NBC is going to tell Notre Dame to keep having a mini-partnership with? The Big East for a few games/yr, or the Big 12? There will be a lot more viewers for ND-Texas/OU/WV/Texas Tech/Okla St compared to ND-UConn/Cincy/SMU/Air Force. NBC is going to nudge ND to semi-partner with the Big 12 over a lesser Big East.
What about from the Big 12's perspective? When Tier 1 comes up, don't you think NBC-Universal will be a lot more interested in being a serious bidder if ND is a semi-partner? NBC wouldn't just have the rights to Texas-ND in South Bend, but also in Austin if they buy the B12 Tier 1 rights. Even if NBC didn't win the bidding, they drive up the price for the competition. And even if NBC didn't somehow seem too interested in bidding, there'd be more nationally marquee games for ABC to select with ND being a semi-member, which would drive up the price of the contract.
Regardless of whether you think ND should be a semi-member or not, you have to admit that financially it'd be a big boost if it happened.