My acquisitions fall into three general categories:
1. I have (for years) bought the kits I missed decades ago when I had less ability to acquire them,
2. I buy virtually every new issue that even slightly interests me, because I don't want to miss them while they're available (see #1),
3. And, I buy everything I can get my hands on that fits into my preference niches.
This pretty well sums up my hoarding, too. I cut my collection way back a couple years ago when I sold off about 500 kits - I probably have at least that many left. I've managed to slow my kit buying down a lot since then, too. The only reason I sold them was lack of space - I'd completely run out of room and it was becoming a real problem.
As Danno mentioned, for many years I couldn't afford to buy all the kits I wanted, and by the time I was able to, most of them were unavailable. So if something even remotely interests me, I pick up one (or two), since there's no guarantee that I'll be able to if I wish to at a later date.
Since my build rate is about one finished model every three or four years, I should be in good shape till I'm about four thousand years old.