That's interesting that they did that, although it doesn't seem very accurate. It's kind of strange that they would talk about their rare songs, but then leave out the #1 most obvious rare song. It's like they are trolling Sakura Iro. Sakura Iro is so rare they forgot it exists.
I agree with Hallelujah though. Natsu Sora Dash was the b-side, but it became way more popular. I like Hallelujah though, I kind of wish they would retire Let it Go. And also Shojou Hiko, because d-lugging is too dangerous. I don't think Hareru yo and See You Again are rare at all. I guess Hakana is rare because that song and Starting Over were the last songs from Check In that their choreographer worked on.