If you choose New Topics or Weekly Summary, you'll still get updates, but the updates will only be related to new forum topics. This works for groups where forums are central to group communication, but where users don't necessarily want to be bombarded with notifications of everything that happens.
Daily Digest and All Email will include information on activity items and comments.
In the future, we may consider making an option that allows admins to remove certain subscription options. But it won't happen until I get a paying gig where such work is necessary, or until I have a few hours to do the development pro bono.
In the meantime, if you're unhappy with the way it works, feel free to hack the plugin a bit, or to create a language file that explains the options better. If you write a patch, you're welcome to send that along to me.