I don't know why you bring up Wii Fit. It isn't required for most games and most of the commonly perceived best games on the Wii don't even use it. If the Kinect was priced closer to $75, it would probably sell like crazy. But $150 for something that does pretty much the same stuff as what you already have is too much.
I use that as an illustration of one of the several additional peripherals needed to enhance the user experience of the wii that is not needed after you purchase the kinect. I know the kinect is not a gaming system and is, in fact, a gaming controller so that argument is not lost on me. But the fact remains that the wii is NOT as inexpensive as you say if you want to take full advantage of its potential functions...they simply charge you ala cart.
But again, it's not needed, and most of the best games don't need it. I don't have one, and have never wished I did have one. You still get all the motion control without it. Kinect, on the other hand, is required if you want motion control. Wii Fit is only optional, and you won't be missing out on much if you choose not to buy it.
Everyone is making a lot of assumption on who plays a wii vs xbox, including myself but I think many households have both...we do. I think MS is simply trying to make a better mouse trap. Just because one company has the market in an area does not mean you do not try to break into it with a better product. Pricing can be a dicey issue and I think $150 is a good price point. $75 more than likely does not have much of a profit margin AND implies lower value (it is well known that many people will not purchase something if it is too inexpensive).
MS has a challenge that I do not think they will completely overcome. Many people simply want them to fail. The reasons are many but they do exist and that alone will create many nay sayers. I prefer to look at this one as a potentially great product and will try it BUT who knows...I could end up with egg on my face as well.
I own both, which is all the more reason why buying the Kinect would be a waste of money. Why would I spend that much money on something that does the exact same stuff as something I already own? I didn't say that MS would be making much money if they sold it at $75, but if they want to move significant numbers of it, the Kinect needs to be lower priced. That's still pretty expensive for a peripheral, and I don't think it's low enough to imply that it has low value.
I predict that in its lifetime, the Kinect will sell enough to break even on the ridiculous amout of money MS is spending to market the thing, but little more. Within a year of release we will see a price drop. But that's just my guess