What I mean is, does it do anything more than just clicking Delete would do (apart from moving the selected message(s) to my Deleted folder rather than Spam folder)? I've been using NIS for months and had thought that it would get "smarter" and better recognize spam if I told it about all the spam emails it failed to recognize (PopFile does a fantastic job of this though it is much much slower than NIS).
What I usually do is select all the missed spam, click "This is Spam" and then empty my spam folder. If telling NIS that a message is spam doesn't improve it's ability to correctly recognize spam in the future then there's no point in my doing so. I might as well just click Delete.