If you ever read Frank the Tank's blog, he does a great job spelling out why Missouri had to make the move.
It's not just about money (although the school should get more). It's not just about stability (which cannot be understated given how Missouri was this close to being left in a conference wasteland). It's obvious something was/is rotten in the Big 12. It's why Colorado left. It's why Nebraska left. It's why Texas A&M left. And it's why Missouri left. And it's also why Oklahoma twice made serious overtures to the Pac-10/12 in the last year. Every other school (except Texas) in the conference isn't wanted by themselves, so they have to take it. But those that could leave did so.
People claiming Missouri will get stomped in the SEC seem to forget Missouri wasn't very good in the Big 12. I can't recall them ever winning a football or basketball title. No one really seems to care about the other sports, but I don't think they won many of those either. So it's hard to go down from where they are. In fact, given Tennessee and Florida's post-Meyer struggles, Missouri could do decently well in the SEC East short term.
Does it suck to abandon rivalries? To an extent. However, Colorado and Nebraska (both former Big 8 members) already left. Missouri may be able to establish good rivalries with border state schools like Vanderbilt or Arkansas. And while losing a Kansas rivalry would suck, that's really on Kansas. Missouri seems very willing to continue scheduling the game, both in football and in basketball. And while Kansas is currently taking a petulant approach to the whole conference realignment, the ball's squarely in their court to continue was supposed to be a big rivalry. If Kansas does want to continue it, it's their choice. But it also suggests the game really wasn't that important it Kansas says no (I mean let's be fair, would Notre Dame and USC stop playing if Ntotre Dame joined a conference?)
In the end, Missouri should be fine and they won't have to worry about Texas leaving them in a third rate conference leftovers.