@framah While breathing might be a bit of a misnomer in the literal sense of the word, environmental equalization is a desirable quality. For a while, I was using kraft paper as a backing on my client's canvases, and I found that it created some issues. I'm in Hawaii, and many of the galleries here are waterfront, with both air conditioning and sometimes extended periods of open air exposure to ambient conditions. This creates radical gradients in humidity (+/- 30%), and to a slightly lesser degree, temperature (most of the time +/- 15º). The captured space inside a sealed gallery wrap always lags the environment outside it, causing an imbalance. I found that this caused issues with canvas tension that varied significantly; issues that went away when I left the backs open. I also found that backing the canvas tended to encourage mold behind the canvas, whereas unimpeded airflow did not. As a result, I'm going commando on the back now, and focusing on just making them as clean and professional as possible for the "eye wash" involved when the client looks behind them.
You ridiculed Jason (misattributed as Hugowolf) for touting the look of his quality stretcher bars, but I think a significant part of the intangibles involved in the art buying process is the appearance of the back side. A beautifully finished back is never going to sell anything, as the front is still where the focus is. Subconsciously, though, seeing what looks to be high caliber craftsmanship when a piece is pulled off of the wall reinforces a client's perception of their investment being worthwhile, and may make them feel better about having laid out a wad of cash, as well as slightly more inclined to buy another "quality" piece of art later. A tidy backing is an easy and traditional means of getting there, but not necessarily the best way or only way of pulling that off.
One more point of rebuttal: In practice, the issue of protecting a canvas from being damaged from behind is quite a limited issue, as a very small portion of a piece's lifetime is spent in transit compared to on display, and there is an equal chance of something poking it from the front as the back. I find the practice of backing a framed piece with glass or acrylic on the front to be a different animal than an exposed, stretched canvas, because the traditional frame creates an entirely sealed, largely impermeable space, that differs from the gallery wrap scenario, so I am not advocating the abandoning of backings for normal framing.
A caveat: I've never used black Tyvek, and the fact that is is marketed as being "breathable" (DuPont's term, not mine), may mitigate some of my concerns about environmental gradients. I would suspect that it would pose no problems in a more traditional environment. For me, in mine, I would expect it to be less problematic than paper, but still less desirable than unimpeded airflow. YMMV