ISS needs 7 tonnes of fuel each year to stay in orbit, so it would need yearly refueling runs just to keep it were it is (or expensive modifications to get larger tanks, I think it only has enough capacity for a year right now). And if you deorbit it you can be sure it's safely gone, and stop worrying about defects that have it crashing down somewhere random anyways - you certainly don't want to have to go out and fix it.
For a decade or so that might be worth it, but on a more long-term timeline it would be more efficient to just launch what you need when you actually need it.
EDIT: what I didn't think about is that one maybe could push it further out and reduce atmospheric drag and thus fuel needs that way, but I suspect the safety aspect still wins.
The ISS is largely where it is due to a set of messy technical and political compromises: it needed to be in an orbit that both Soyuz and the Space Shuttle could reach. Because they both launched from places that aren't on the equator, all orbits reachable by them would be inclined, so there's only a limited set that both can get to. (Without doing a ruinously expensive plane change manoeuvre.) And of course the higher the orbit, the more fuel you need, which means less payload, so it ended up in quite a low orbit.
Now the shuttle's gone, there's no realistic proposal for delivering heavy components to the station, so it could be reboosted into a higher orbit. You'd end up spending less fuel on stationkeeping, but more fuel on routine deliveries and crew transfers.
I really have no idea whether this would be worthwhile. Anyone?
For a decade or so that might be worth it, but on a more long-term timeline it would be more efficient to just launch what you need when you actually need it.
EDIT: what I didn't think about is that one maybe could push it further out and reduce atmospheric drag and thus fuel needs that way, but I suspect the safety aspect still wins.