I am opposed to this being added to the plain text version of comment notifications (aka, the version I use). In that case, it's necessary to open a new browser window (or tab) anyway, and the only benefit is that the page would come with the "mark as spam" option pre-ticked. This is a tiny benefit, and not enough to overcome my reticence to having Yet Another link at the bottom of emails. The plain text versions can get somewhat confusing at times anyway, since they alternate link and description on following lines, and it isn't always immediately apparent whether a link matches the description that comes before it or after it. Adding extra options to the list will only worsen that problem.

So, I'm opposed to this being added to plain text notifications, and entirely indifferent when it comes to HTML notifications.