The "-1 card attack" is one of the better discard attacks. It is appreciably harder to fish for an engine when playing a stock village does not draw a card. This can be crippling on a few turns where discarding two coppers is meaningless.
A second Relic is moderately useful for Kc/Scav/Relic if the other guy is not trying to set up a double Kc/Scav deck. I would not burn an $8, $7 and likely not a $6 on Relic, but I could see it being useful for slowing some big engine (e.g. Goons with really crappy draw for the Kc) while you ate the provinces. Another place where a second copy is helpful is in Adviser decks where the attack is your most painful thing to hit the other guy with. An even more edge-casey option is with Storyteller/Black market & Gov/Cr/Lost city/Soothesayer, I could see Storyteller/Soothesayer working well with 2 or more Relics to make the other guy lose multiple draws.
Relic works phenomenally well with Mine. It is absolutely worth it to turn Silvers into Relics with Mine when you do not need Gold's buying power.
It is also handy to tuck a Relic into a Venture deck if the other guy has not gone Venture.
Rabble makes for a brutal combo, particularly as part of a Rabble engine - 3 Green on top, draw 4 means that you either have to get insanely lucky to kick off an engine ... or you have to hope that dead drawing does something for you. Losing 1 in 5 cards is a good attack, losing 1 in 3 or 1 in 2 useful cards is stupidly strong. Even Rabble's weaker cousins - Spy, Fteller, and B-crat all amp up the -1 token and have some serious increases in firepower with a -1 Token helping out.
Relic is better than average soft counter against Fools gold and Tmaps. The odds that 1/3 cards is not a match is typically a good bit higher than 1/4 cards not being a match. Likewise, it is a nice counter to low/no draw combo decks. A Bishop golden deck cannot hit provinces (barring Plat or cantrips) off 4 cards. Other combos, like Kc/Remodel (no draw, but strong trashing) also crash and burn
For straight BM games, it is pretty helpful as it easily wrecks a lot of $8 combinations, slows the other guys cycling, and doesn't compete with gold. Particularly with non-draw BM this would likely be my goto $5 whenever I am not looking for more terminals.
At the end of the day, I would pretty much need to have a good reason not to grab a Relic in an engine (e.g. Jack, Pool). The potential to cause a whiff in the last couple of turns is just too strong once my engine is built. -1 is a delayed Minion which just all manner of good. +1 action, $2 -1 card is at least as good as Militia and certainly respectable.