Anyone else notice that there have been a number of manga based off of harem or harem-like series with female protagonists popping up recently?

To Aru Majutsu no Kinsho Mokuroku -> To Aru Kagaku no Choudenjihou
Steins;Gate -> Steins;Gate - Boukan no Rebellion
Hidan no Aria -> Hidan no Aria AA
Itsuka Tenma no Kuro Usagi -> Itsuka Tenma o Kiru Majo (no scanlations yet)

There might be more examples, but that's all I know. Often it seems that these spinoffs tell the same or a similar story from a different character's point of view. Anyone have thoughts on this? I've seen comments from people who prefer Railgun over Index, and others who started with Index don't care much for Railgun, or they like both.