I think it's good in big money situations (particularly in the early game) where there are no really good 5s and/or you can't spare another terminal action. In those situations, copper is not really bad and cache, which is basically gold often translates straight into more gold or provinces.
In a way, buying cache is similar to using steward for +$2 to get an early gold. You accept that you have 2 "extra" coppers (since with steward you would have trashed 2) to get that early gold. Of course, buying cache is a little worse, since you're also passing up to opportunity to buy an alternative good card.