Good question! Actually, I think the real party is the all-H!P tour. As for this unit, I think I have found the meaning of it. Kaori, Maki, Nacchi and Koharu are the only ones that have had a solo debut after joining the group. A real solo debut with major CDs on the shops. (Except Yuko, she's the only one I miss, I said it; She might have refuse it) So, from a marketing point of view, it's pimpage for everyone. Is it fair? Probably not. But knowing how Tsunku is, I think none of the girls is surprised to see this. A reunion is a reunion, if you can't have it, don't make it. Asuka, Sayaka or Aya -among others- wouldn't show up for a stage performance nor a single recording. Probably they would be on for a MTV or VH1 documentary, but not for a song.
And yet another point related to the last sentence, it is that the kind of reunion in H!P limits already happened in Kohaku 2005, and it's not like Japan tuned-in the tv to watch their old idols. So why bring that on a CD giving two words to each person? Nah, they have preferred to make it a four girls group, all four with solo affairs, and something new that tv shows can be attracted about.
I am so curious about the fact that Neo Wings says "10th anniversary begins". I wouldn't be surprised if this special unit does more than one single this year, and is used in every show to talk about MoMusu, as well as random events. Like I said, they'd be not only pimping an anniversary, but everyone of the MM soloists.