IMO the tier system is just a reference/guide to which girls are considered to be popular and have more content.
Should it be split again into more sections?... a waste of time and effort.
Should all girls get their share in new content?... Yes, but it is still a "work in progress" game, so could be closer to that in the end.
If we had each update having a piece of content for every girl, that would take a lot of time and be large gaps between updates.
2 of the new content for Yvette were delayed from the last update, or that update would have taken longer.
Yes, this is more a Yvette update, but it could be a Faye, Heather, Jessica.... more focused update next time.
IMO the more important thing is that the content is relevant to the girl, gives you more about her and her backstory.
No point giving all the girls the same content, as once you have completed one girl, you would really have seen everything.
Also, a large time of the devs work at the moment is in upgrading the quality of old content, and also giving them a more personal feel to be about the girl in them.
It's very much still a "work in progress" game, comments about it being equal etc... should really be made towards the game completion... whenever that is?