Yeah the parity axe swings both ways, it doesn't discriminate. It can strike in subtle ways as well, like I'm sure the non hdd 360 sku caused much grief by year 6 leaving the parity axe to strike both the ps3 and 360+hdd setups. Honestly you aren't hugely concerned with it in the beginning of a gen because you are so busy just trying to skip the damn game due to the joys of console development forcing you to re-write everything from scratch when a new box comes out. It does get irritating though after that first game is out and you have more time to actually think about things you want to try and implement.
Yeah, the none HDD 360 SKUs must have been hurting everyone by the end. I'm surprised MS were able to force all but a tiny number of games to keep the none HDD SKUs supported right until the present day.
Those optical only models must have kept bringing in the customers and their dollars at the low end, especially with Kinect. Too bad when you consider what a huge upgrade even a $10 USB pen drive can be...
So yeah, the parity axe swung both ways last gen and it's going to swing both ways again this generation.