Noting that in Adventure mode you can't (or couldn't, I may be out of date/misremembering) use coins of a different civilisation except possibly at the level of barter items, so I'm not sure that your outpost (striving towards becoming a Royal Seat Of Power) would necessarily be expected to be more lenient towards them.
But I like the idea of coins. Passing trade (traders should always want to trade, but perhaps there should be a distinction between Civilisation Caravans that circle the area to promote the interests of the local Elven/Dwarfish/Human civilisation and more sporadic traders that are more akin to Tinkers, or even Rag-And-Bone men) that isn't hostile in some manner[1] should be given shopping rights, perhaps even "I'll swap you this axe that I found for that food you're hauling and that fine hat you're wearing" possibilities with individual dwarfs it encounters (controlled by a mini-Trading dialogue that becomes available if you have the "Allow impromptu bartering with strangers" option enabled).
But I think the main controversy is the coins. At least at lower skill levels (though I've only minted them as an experiment, way back when), a coin appears to takes up the same unit of materials as a metal throne, table floodgate/whatever. So a more realistic material quantifying/division system is probably a prerequsite for coins that can be expect to be slipped into (and possibly pickpocketed out of) a handy purse to little effect upon the stamina of the dwarf concerned.
I'm thinking that this issue is being addressed, but haven't checked the "Next version: Tasks to do"-type thread recently, so I'm not sure if it's specifically on there or not.
[1] But opening up the possibility of spies/sabateurs coming and going as "normal" trading friendlies but either giving later waves of hostiles an idea of patrol military routes/more efficient pathing information, or turning "thief" or building destructor while in the heart of your fortress (or, more likely, in amongst your trading area, initially limited to just the Depot, the odd outlying workshop and the import/export stockpiles... at least until they sneak across the lava-bridge).