I don't agree with the notion that a tv tuner would take away from the iTunes market simply because the way in which tv programming has changed. If you look at the way a season is being aired and how viewers are watching content, it represents a flow:
Shows are not played into summer rerun mode because there is this need to compete with other networks that offer shows specifically based on their status as fresh. This has been noted in various media outlets as a trend and not a fad.
With the lack of immediate repeats, a season is run only once until you have to wait for syndication to catch up. At this point, this is where the iTunes/DVD market comes into play; if you missed recording the shows, this is your last recourse (legal mind you).
In regards to iTunes, it's a volatile format compared to hard formats such as DVD, and to a lesser extent, VHS.
In this whole flow, I think I have created some questions that I think would really determine whether a tv tuner would hurt or work exclusively to the iTunes market:
How many people actually backup their iTunes purchases?
Is it a viewing habit to pickup a show in mid-series run? Do people go back to see the unseen portion of series? If so, is it through DVD form, syndication, or both?
Would people purchase upgrades/external storage in order keep all the on air tv they record and iTunes material or would they purchase hard copies that contain additional features?
Imo, all these questions have responses that point to the TV tuner not being an influence to the way in which iTunes would be used by the TV Viewing Audience. The VHS did not kill syndication and the DVD will not kill hard copy/iTunes. And I highly doubt that when it comes to shows that people enjoy, they chose iTunes over DVD's which offer much more. To me, iTunes serves as a quick fix to those who want programming that isn't avaliable in other formats or choose to have only snippets of items that they don't enjoy on an entire level.
I hope this didn't seem like a rambling post, but I wanted to really engage into a discussion on this topic because with the way in which dvr's have become prevelant, the TV landscape has changed in ways that I don't think has been fully understood.