That is one of the WORST kind of endings for a romance story IMO. They did that in Insomniacs and I absolutely hated it. What ever is the point of an open ending for a romantic story when the pairing is in fact the natural resolution of the story? It feels like a bait and switch. Imagine Dress Up Darling having an open ending. The actual ending was already disappointing, but I promise you people would riot if they left the ending of Dress Up Darling "open to interpretation."
If the alternative is making one of the girls left out of the trio after making her fall in love and get close to the guy, then yes, open ending is far more preferable than choosing. And harems are often not very serious love stories, so many of them are just fine with not ending on choosing any one girl. Rejecting one girl is good when it's a part of story designed from beginning as bittersweet love triangle romance.
Dress-up darling is completely different kind of story that can't be really compared to this one, one about couple growing closer to each other , so it should have couple ending, but stories about friendship couple can actually well told despite open ending. In this manga's case the best ending is throuple since they all like each other as best friends, choosing only one girl is just sad downer ending with one girl relegated to third wheel status, it would just suck. For same reason I don't want multiple endings, they are feel like cop out anyway.
