"Don't most people voluntarily buy a new copy of Office with each PC anyway?"
Not true at all. For compatibility reasons, it's sometimes easier to keep an older version than to get the newest version (as an example, excel 2008 for mac dropped VBA support but it was brought back in 2011)
Do you have any data on that? Because I've seen a few (sample size ~4... so mine isn't useful data either) non-technical people doing this and in pretty much all of the cases what they wanted was to keep using the same software they'd been using for the past X years. Getting their "computer guy" (usually me, or some local teenager) to copy files over for them was their preferred way to move onto a new computer.
No hard data, but it's what I've seen the people I know doing. Lately they buy Macs, but when they buy Windows machines, they usually choose the pre-installed Office because it's easier than installing it later.
It's hard to buy the vendor lock-in arguments, either, because Office has used proprietary formats from the very beginning.
I don't use Office, though, so maybe I'm missing something.