Many artists draw Fate & friends as slutty bitches, so I guess there's a demand for such content (look at cyclone & TRI-MOON works). Besides, I and probably not only me don't mind slutty heroines as long as their 'adventures' as well-drawn ;].
I was under the impression that Mikuni Mizuki was a fan of Fate first and foremost, but it's nice to see this artist branch out into the other characters from this series. Fate seems to be having a fun time telling Signum what a part of married life is like.