Re: What is the purpose of the common paper backing on a framed oil painting?
There's good & bad reasons for it. In the case of stretched canvas, a dust cover makes it inconvenient to access the corner keys for adjusting tension. However, it does help protect the back from dust, cobwebs, insects and other stuff that comes along.
Anyway, dust covers are not strictly necessary. They do make the back look finished but have limited protective value, especially paper covers which can easily tear or lift on the edges. Stretched canvas creates a hollow space inside the stretchers, making it inevitable that a hole will get punched in the paper.
It's common practice to cover the back of stretched canvas with a protective board (mat board, coroplast, etc), which does more to protect the piece than paper alone. It also eliminates the hollow space.
As a rule, I cover the back with board and finish with black Tyvek, which is much less likely to tear.
Last I knew, standard practice in some other countries (England, maybe others) was to not bother with a dust cover.