Romney should not spend in MI either. Same reason. He wins MI after he gets beyond 300 EVs. Meaning it's gravy.There are a LOT of swing states right now (11 within 5% according to RCP), especially after Romney's pre/post-debate surge. When the polls tighten in a state that you thought was previously out of reach, it's tempting to flood the state with ads and other money-hungry campaign activities. However, in terms of winnability, it makes more sense to go harder in the closer states that get you over the 270 electoral votes required to win. Sure, it would be great to win Pennsylvania, Michigan, Minnesota, Wisconsin, Oregon, but they're not necessary. Since there are so many other states closer to Romney's reach than those, enough to get him to 270, that's all he needs.
According to RCP averages, there are 3 states in which Obama leads by less than 2% right now: NH, VA, NV. If he gets those, he already has 267 electoral votes, with OH, IA, WI, MI, PA remaining, in order of accessibility (2-5% Obama leads in each). Sure it would be great to get all of those, but with just one, he crosses 270 and wins. So barring locally mitigating circumstances, it's much easier to get the one or two with the smaller margin (OH, IA) than to shoot for the moon and go for all of them or the big ones. Doing that means diluting resources in states that while leaning his direction could slip from his grasp if the campaign lets them.
Only when you're absolutely certain of victory (270) can you really let down your guard and go for the whole shebang (337).