To be honest I am quite happy that we got less Mer this season. In the last couple of seasons she just drove me crazy and in the last few epis of season 4 I started liking her again and I think that if Shonda had given her more screentime this season that would prolly have meant more drama, which would have driven a lot of people crazy again.
and as for 'Izzie's Anatomy' I understand that Mer fans are quite pissed that Izzie got so much attention this season and I am too pissed at Shonda for trying to shove Denny down our throats but I think that the attention she got after Denny was gone was just something that had to be. The woman has cancer, all her friends are worried about her and of course that means that a lot of things are centered around her well being. Or would you have preffered it if Mer was the one to get cancer?