When you have a membership site, you do not generally want to inform subscribers of new posts etc that are in levels that they can't access. At least it is annoying to the lower-level subscribers to see information about posts they can't access and there may also be security issues in the sense that you don't want them to know about a new post.
Subscribe-by-email is a plugin which effectively replicates Subscribe2. There is very little difference between the two as far as I can see.
To make it much more useful, it should also check whether a user is allowed to see a page before sending information. The option should be there to send emails to subscribers at certain levels about higher levels but it should all be under admin control.
This would make it invaluable for membership sites.
The same could be said about tag clouds BTW. We need a tag cloud widget that is integrated with the membership so that tags apply only to 'higher level' posts are not seen by the masses....