@Biswapriyo Using a webapp to do it and all it gives you is a link that fails? No. Using a program on your computer or possibly a web browser addon? I'd say sure.
@Biswapriyo Well, yes, in theory we shouldn't encourage it for that reason. But we would probably need a meta question to specifically decide if we close them outright as a violation of the ToS. It's a bit hard to specifically mark all "video site downloaders" as off topic though and if a program happens to deal with Youtube as well as others which don't have as restrictive a policy then it's difficult to police.
Unless it is a clear call for warez, discussion of DRM should be given the benefit of the doubt.
Per Joel:
I would hate for us to censor legitimate, technical discussion just because what we're talking about happens to be illegal in some jurisdictions. We should have the same standards as un...
Both Jeff and Joel weighing in (well, Jeff quoting Joel)
On SU such things are more likely to be off topic as they are often web services
But questions about youtube-dl are potentially on topic