I'm in a speculative mood right now...what if Katie Holmes is smarter than she appears. Supposing that the Tom-Nic marriage was really a business relationship, Katie may be thinking that if she puts in 5 or 9 years with Tom, that'll give her exposure and connections. Then she can divorce him in a dramatic break-up and go on to make pretty good movies. Nicole didn't really do anything all that spectacular before her marriage...good (like To Die For) but probably not enough to make it absolutely huge--more independent or period pieces. She was probably on track to have a career path similar to Naomi Watts--better than I'd ever have, but still not top of the heap. But with the right connections and the right media spin, she's done really well. Got that Oscar Tom never did, at any rate.
But then, maybe Katie is just a twit...though I really did like her in Go, The Ice Storm, and Wonder Boys.