I'm not sure "leeching copyrighted content" is a fair description of what Youtube-dl does. Yes most of the content you will download with it is copyright, but it is content that you already have a right to see, and in most cases the expectation is that you would maintain your right to see it for as long as Youtube (or other sight) remains active. The main difference is that Youtube-dl allows one to view the content with a program other then the browser. I suspect that few people uploading to a video sharing site did so with the intention of requiring people to view it using that sites player, but rather did so with the intention of people viewing it, and the player restriction was incidental.
The one place I can see where this breaks down is in advertisements, but I consider that to fall into the incidental results. (Although Youtube-dl does have a --include-ads option)
>it is content that you already have a right to see //
You have opportunity, that's not the same as a right. The content supplier is under no obligation to provide content to you, ergo no "right to see" that content.
That said, personal time-shifting and format-shifting should IMO be a normally allowed part of the copyright deal.
The one place I can see where this breaks down is in advertisements, but I consider that to fall into the incidental results. (Although Youtube-dl does have a --include-ads option)