@harry76 For your ISP, it becomes more of a legal protection. No ISP wants to be known as the ISP that allows copyright infringement on their network. In the case of a user who gets a multitude of requests, this obviously increases the risk that the copyright troll can start litigation against the provider for allowing such activity on their network.
I personally do not use a VPN when logging into the site just to browse or use the forums, however on the machine that I use for the actual torrent traffic, I absolutely use a VPN. I know that in the DPI state of Canada, my ISP can most definitely see what I'm doing behind the VPN, however it is not my ISP I am worried about. It's more of a protection so that nothing attaches my actual IP address to any torrent traffic so that my actual IP address does not appear in the peers pool. This way, my ISP can most definitely still see what I'm doing, but potential copyright trolls cannot tie my identity to any specific torrent.
The other added benefit of using a VPN is that if I do hit on a torrent that is being monitored by copyright trolls, they'll have to approach my VPN provider for further information, however my VPN provider has a strict no logging policy, so they are not able to disclose information that they do not actually have. This saves me from having my ISP account tied to any of the torrents I may have active at any given time, which both protects me from being tied to the torrent, and also stops my ISP from receiving any potential notice.
It is generally assumed that although in a DPI state such as Canada that the big players to the market can see what kind of traffic you have going even through the VPN, they typically do not give a damn what you're actually doing, as long as they don't have copyright holders knocking on their door with infringement claims.
That said, depending on where you are in the world, it may be a wise idea to use the VPN even when logging into the site, just to make sure that even the sites themselves cannot tie your actual IP address to your account. This is particularly helpful if the security of the site or its staff are compromised somehow. A person may be able to tie the VPN IP to different torrents, and may even be able to tie your VPN IP to your account, but if they are not able to sniff out your actual IP address, then the link they need to get your subscriber information is missing, thereby making the VPN IP address a dead end (assuming that your VPN provider has a no logging policy).
