I wasn't a big fan of the pirate arc either (as I am famously not a fan of the pirate twins - or anyone in that arc) but at least I felt like it had a purpose. I was doing it for Eliza. This just seems like Buu didn't know what to do to make the story longer so he dropped us into a completely different game for awhile.
The difference for me was that the pirate arc was basically Kira (a couple of scenes with Kelly, and a few fleeting glimpses of Pearl, otherwise it was all Kira). If you didn't like her, then it was not enjoyable.
For the most part I enjoy getting to know other girls, so on the island I get to hang out with a heap of different of girls, some I like, some I don't, but at least I get variety.
I get that if you are playing a solo Eliza (or Luna etc), that everything is going to be annoying while you're away. Eliza is my definite favorite, but there was quite a few others that I like a fair bit too, so I just accept that this is how Buu is going to do it, we're going to be collecting girls and allies for the foreseeable future.
As to the purpose of the Island, I'm only guessing, but it feels like we are collecting a few things here. Some powerful artifacts, some allies (I suspect that now we've become
hunter partners with Vex, she might be coming home with us, the others to follow when needed for battle). But perhaps more importantly, we're getting gods on our side. Gaia and perhaps Malanthe too.