I'm playing games since 1992 or less and alway the generations was defined by the similar power of the consoles.
So the guys are just changing that to fit Wii and Wii U.
My personal division...
Gen6: GCN, Xbox, PS2, Wii
Gen7: X360, PS3, WiiU
Gen8: X720, PS4
Wii is in the middle of the Gen6 or Gen7 so you can put it in any for fit your pourpose... the Wii U the same thing in the middle of Gen7 and Gen8.
I put they together the precious gen but I can't disagree with who puts them in the next gen.