Let me add my own rambling on this subject...
While I feel that even the most unpolished, rawest members the 9th, 10th gen members never dragged the group down I must admit that for the most part it was because they joined at a time when Morning Musume as a group itself was in the middle of another transition period with the last of the Gokkies graduating and to be rather blunt... from Maji de Suka Ska to Renai Hunter there wasn't exactly many challenging materials for 910kkies to work with. Which is good, really
, it gives them time to get up on their feet and grow to the point where now they all capably shouldered their parts in the group with Fukuhime, Riho, Ayumi and Maachan as the pillars.
Sakura joined right after 123 and Wktk when Morning Musume has just begin to distinguish a new style for themselves. As the sole addition in 11th gen, she has done remarkably, astoundingly well by not only keeping up but also contributing immediately to the group with her voice. She got along and blend in with the group surprisingly fast too.
Now it's a very different period for the group. They had established a style for the group and have a clear direction going forward, they have target,s goals, ambitions and they're very hungry to reach it.
In my humble opinion it really is not the time to slow down and add new members
unless they managed to find a truly outstanding candidate. This has always been the mission from the start iirc from Tsunku and Sayu's interviews and radio shows, to find girls that is so cute and so amazingly talented they could join Morning Musume now and contribute immediately to the group's future.
Lastly,
The well-received new direction that the group showed with 123 and Wktk and has since continued to gain even more momentum with Help Me! and Brainstorming's oricon weekly #1st was in no small part due to the intense formation oriented choreography. How intense? Well, two members was benched from injuries and reduced to crutches in the latter half of last year and during concert tours the sight of many members with bandages around their ankles, knees and thighs became more and more frequent. I'd really prefer to avoid a repeat of those scenes that but can we honestly expect untrained newbies to come out of their lessons unscathed and immediately master the recent demanding backlogs or keep up with future releases without dragging group down?
Add to that the other usual considerations for adding new members (work attitudes, personalities, looks etc) and I'm not surprised and actually a little bit relieved at the result. Maybe in a year or two when MM is not busy capitalizing on their recent momentum they could a afford to slow down and let fresh new girls with untapped potential grow in the group but until then....
Thanks for keeping Morning Musume as it is now for the time being Tsunku!
/end rambling