This is also the reason why those uppity gays are not content with the crumbs of "social unions." Non-married couples do not have the same rights as married couples.
of course this is US-centric
As a sibling commenter noted, the trick is then getting the hospital, partner's family, etc. to recognize the PoA document; while the law surrounding powers of attorney is generally quite clear, there are certainly plenty of horror stories of hospitals, families, etc. ignoring such documents. I imagine that the likelihood of this happening varies from state to state, hospital to hospital, possibly even doctor to doctor, and so on.
That said, in the US, basically all of the time, you'll be _far_ better off for having drawn up a durable power of attorney than you would be if you hadn't. Definitely money well-spent if you're in an unmarried partnership. I suggest finding a lawyer who specializes in LGBTQ family law; they generally will be happy to take any couple on as clients, regardless of whether they are same/opposite sex, cis/trans, etc., and they have literally "seen it all," which means that they know about all kinds of legal edge cases that can cause trouble for people in a non-married partnership.
As an aside, I'm continually amazed by how many _married_ couples don't have wills, PoA documents, advance directives, etc. etc. filled out. Even if it's not strictly necessary from a legal perspective, having one's wishes and instructions written down explicitly is a relatively small-seeming thing that can make a very bad situation noticeably less awful (or so I've heard from people who've been there).
Here's a big one: if you work for any church-based organization (and they are huge employers, e.g. operating large schools, hospitals and universities), they might fire you because they suddenly decide you are violating some arcane religious doctrine.
Now you might say "I didn't want to work there anyway," but that's not how life works for most people who are not well-paid twenty-something software engineers.
Sure, it depends on the law in your local area. As non-married couples gain rights, it removes reasons for people to get married.
Whatever additional rights you get for being married are your primary motivator: that's what I was trying to say. Here, it seems primarily to be EU work visas.