For me, the issue is none of these cards happen in a vacuum. In a slog game (say, margrave and no villages) You're only going to be able to sustain 2 terminals early, and 3-4 terminals late in the game. Of course you pick up margrave or smithy or whatever hand size increaser at first, but cache becomes useful if you couldn't hit 6 and you don't care about the extra copper so much since it's a slog game. I love cache with trader or watchtower, it works well enough with horn of plenty and fairgrounds while still providing buying power, it can be a great buy with gardens.
I feel that a lot of analyses miss two things, first that a card isn't useless even if you can't build a whole strategy around it, but some cards in dominion are just generally helpful: at the right time in the game, a smart buy will make the difference between winning and losing, but because the effects of other cards are stronger the subtle effects get ignored. In the case of cache, if your deck can consistently hit 5 but not 6 and you already have enough terminals in a slog game, you could consider cache. I'd consider it to be about even with silver, except that it increases the variance of your deck, which can be a good thing.
Which brings me to my second point: Averages are nice but you can't spend too much time looking at averages, because in Dominion a single lucky turn can make or break the game. So, while you can't rely on a lucky turn to happen, you can build your deck to increase the chances, or at least make it possible. I think it's a mistake to assume that when someone 'gets lucky' and pulls off an amazing turn, that skill had nothing to do with it- they had to build their deck in such a way to allow that turn to happen, gambling that the game would last long enough for them to get lucky eventually. Cache is a card that can help with that- sure the coppers aren't great, but if you couldn't hit 6 that +$3 may come in handy when you need it. If you didn't buy the card you wouldn't have the opportunity. Granted I rarely buy cache but I don't buy explorer much either, but both cards have won games for me, and I've lost games to cache strategies, I definitely think there is strategic space for it.
on another note, it's not correct to assume that if cache, or any other card was not in dominion, that the replacement for it would have been any better. Both better and worse (and better but more evil (rebuild)) cards have made it into dominion, odds are cache would be replaced by a card with a similar level of strength.