To be honest, I haven't ever held much of an interest in the Eggs. That's not to say that I think they're unimportant...it's just that there's only one me, and I can only spread my fandom so far, so the Eggs are a part of H!P that I don't really feel like getting involved with at the moment. If any of them get bumped up to 'group' status, that's when I start to care.
In Erina's case, I think she's cute from what I've seen of her in Ongaku Gatas, but I don't understand all the hype around her (maybe I just haven't been paying enough attention to the group or something). Lack of knowledge notwithstanding, I was surprised to hear that she was going solo, and even more surprised to hear that she was graduating from Gatas. I think it was a wise decision for her to gradute from the Eggs, but Gatas could potentially take a big hit from her absence. Still, even if they don't, graduating the most popular new face of the group and making her a solo artist after not even a year is kind of a slap in the face for Ongatas, and says to me that UFA isn't taking them as seriously as they should.
I especially feel bad for those members who weren't Eggs before Ongaku Gatas was formed. Hitomi, longtime captain of Gatas Brilhantes H.P. and former leader of Morning Musume, is given her own (well-deserved) group to lead. Ishikawa's main group, v-u-den, is disbanded so she can focus on Ongaku Gatas (or, at least, that seems to be one of the reasons for v-u-den's demise). Satoda has finally been given something to do after being all but inactive in Hello!Project for months. KonKon actually came back to H!P, while still attending one of the most respected colleges in Japan, to goalie for Gatas and join the ranks of this new group. By giving Erina her graduation on top of a solo career, UFA is essentially saying "well, we like you guys and all, but we feel like this girl is too good for you, so off she goes!". It is extremely disappointing to see these longtime members disrespected like this, especially since a couple of them have made some fairly substantial sacrifices so Ongatas could happen.
The situation can't be doing any wonders for the morale of Erina's fellow Eggs either. Think of it this way: the Eggs are basically the 'runners-up' from auditions, whom Tsunku deemed good enough for H!P, but not good enough to be involved with a specific group. It's a big change for these girls when they're put into 'real' groups, and I think that the team dynamic is important for them during a group's early stages (especially for one that's based on something as team-oriented as futsal), as it allows the newer members to get support from each other and their sempai while they get used to being in the spotlight. By graduating Erina from Ongaku Gatas, UFA is marring the unity that these girls need most right now, and I wouldn't be surprised if there were some hard feelings toward Erina from those still left in the group.
And let's not forget about Erina herself. As exciting as it must be for her to begin a solo career, I think that taking her out of an environment she's still getting familiar with and throwing her into intensive training for her own venture is just too much, too soon. I'm sure that she would have benefited greatly from having the balance of Ongaku Gatas and solo training (and, eventually, her solo career) at the same time in her life, as she would have the support of the other Gatas to push her through what is surely going to be a difficult several months, as opposed to having to tackle it on her own. After the training's over, and she's had a couple months of solo experience under her belt, then I could see the choice for her to leave Ongatas as a reasonable one...but doing it now is really jumping the gun on UFA's part.
Please note, I have no problem with Manoeri becoming a solo artist. As a talent agency, UFA has to take risks and try new things, and this could be a potentially great move. What I do have a problem with is how Ongaku Gatas is really getting the shaft here by losing a member so quickly after they've started. H!P members have begun solo careers and stayed with their groups successfully before (Koharu being the obvious example), so it just doesn't make sense to steal Erina away from Gatas at this critical stage in both of their careers. I hope everything works out okay, since I really like what Ongatas has done so far, and I would hate to see this create problems for Erina, but I have my doubts. I suppose we'll just have to wait and see.