Thanks for the info.... just as I thought.
I just think that if they could start having a tie-in single with either a dorama, series, programs or movies, they could have a bigger appeal to the general public. Many popular artists nowadays usually have or had tie-in singles to such j-doramas, tv programs, anime series and movies.
But I guess even without such tie-in, they're still capable of good sales... But if they do have a tie-in, they could possibly increase their sales and popularity in the japanese public.
If maybe Takahashi Ai's dorama is able to have a MoMusu or even an Ai-chan's solo song as a theme or insert song, it could definitely help. it may be a small step, but still a significant one.