Marginal to who? The Big 12 is a conference of Texas and plains states, with an outpost in Appalachia. BYU, Houston, Cincinnati and UConn would all be Top 6 markets in the Big 12 the day they signed up. UConn football is weak, but the other three have tradition, strong recruiting, and solid programs that would be competitive on day 1. UConn, Cincinnati and BYU would strengthen an already powerhouse basketball league. From a TV perspective, I am sure Fox and ESPN would like to pay less money (who wouldn't?), but adding those 4 schools actually makes this league better from a TV perspective.
If we were talking about the ACC, they would be crazy to add anyone but UConn, but this is the Big 12, which has some issues, so there is a reasonable case to make for 4 schools. If the Big 12 doesn't expand, the conference will not exist in 10 years. That is the best argument for expansion.
The best argument against expansion is that Texas probably doesn't want the Big 12 to exist in 10 years. That is a stronger argument.