I like your explanation of epigraphs as being a way of “…letting other voices in” very much, Dave. Also, I would add (also obviously): a way of conversing directly with other writers across time and space. An epigraph can do a lot on the page, too, for the reader: contextualize, mood-set, jar, subvert –
I do feel like I see overuse of them for less generous reasons (seeking ‘legitimacy’ of some kind, usually – too often, it feels like they’re a marker of having something to prove). But that ain’t all of it.
In my own stuff, 9 times out of 10 when I start with one I edit it out in revision, because it was serving as scaffold for what the finished poem (eventually) held up fine on its own. And I admit that they annoy me in other people’s work when I feel like they’re doing the work the poem itself is supposed to do. But that 1 time in 10 when they utterly change the work, make it bigger – in those cases there’s no other way to open that door that effectively.