It really doesn't destroy much either way - NIU isn't really losing any major longstanding rivalry the way an Akron would if they left the MAC, and who (beyond maybe Ball State) has any buzz when NIU comes to town?
If there was one team to lose that wouldn't really upset the status quo either way (despite their success at times), it's probably NIU.
The value is likely in lower travel expenses in the Horizon League for everything else (more than offsetting the increased football travel) plus probably a little more TV revenue. This definitely isn't a risk-free move, I'll agree. But I see why NIU is going to try it.