I really like them! This kind of ensures that each protagonist has to have a defined enough personality so that they're distinct from each other, and the writer can specifically cater each protagonist to fit the love interests she's paired with, resulting in dynamics that are both unique and can make more sense at times. Sometimes, less can be done with a single protagonist and the writer can make her act a bit more inconsistent just to forcibly fit her with every love interest, even if it's clear that she fits with some more than others. So multiple protagonists allow the writer to design them to fit their respective love interests and the dynamics and themes they want - I think romance can be enhanced by having both characters written more specifically for each other so writers have lots of creative control in depicting the relationship that conveys the themes they want the most.
It's also interesting to see the dynamics and interactions the MCs have with each other, if they do, like in Norn9. I love Norn9's MCs and think that they have some of the strongest MC character writing, partly due to how the multiple MC set up required that they all existed as very distinct and unique characters from each other (especially since they interact so must exist as external characters of their own right outside of being only the MC or POV character since they still exist in the other routes where they're not the current MC). It challenges writers to distinguish between different characters' voices when in their point of view, so that not everyone just sounds or thinks in the same way - I really like it when the personality and perspectives and character of a POV character just overflows and shows through in even the voice of the writing and their thoughts and way of narrating, and multiple protagonists usually means that more attention is paid to things like this to distinguish between the characters. Multiple protagonists can also create contrast with each other, both in character and themes, which can strengthen the storytelling (like through parallels and foiling and the interaction between what each protagonist represents, etc). Seeing different perspectives is very interesting, and how they contrast with each other and help to provide different angles on a singular overarching narrative can contribute so much to the overall themes of the story.
On the other hand, I do still really like singular protagonist games as well, and I think that a skilled writer can keep the MCs consistent and vital (as in, it's important that it's specifically them) for each route and love interest - this instead helps to highlight slightly different sides of the MC, which can flesh her out even further while still keeping her coherent and consistent (because I don't really like it if her character just arbitrarily changes for no reason in different routes - a character acts differently in different situations, but they're still a single, consistent character so shouldn't differ in ways that make no logical sense imo, unless maybe it's like some complete alternate timeline thing? but that would still be logical in that case). It's like, through these different dynamics and interactions and story beats, more of her is characterised and revealed to the reader, and having just one central character that the story is centred around can make the main themes (and what she represents, etc) stronger and more coherent, since the protagonist is the most important character in my opinion. This main theme that she represents can be examined even more deeply through her interaction with the different love interests and the things that happen in the different routes. Like what happens when this specific character (this single MC), with the role she has and who she is, interacts with these different people or goes through these different things? It's very interesting and helps to basically reinforce the, I guess, thesis statement of the game (shown with her) through different angles (as well). Having her as like an anchor usually helps to make the themes very tight.
So basically, I like both, but it would be nice to see more games with multiple protagonists (as it's relatively a lot less common), especially if it's like Norn9 where there's interaction between them.