Agreed, configurable makes sense though obviously I'm a proponent of the 
default being False for adding and removing oneself from the Nosy list I 
don't ultimately care what the default is if I can control my account.

A configurable roundup instance wide default for users who haven't 
explicitly set anything is also good.  Different installs want different 
behaviors.

the config option(s) for this should be something along the lines of 
"user/user(s) get notifications for issue updates where no comment was 
added."  or "where a user added or removed themselves from the nosy 
list" or "where an attachment was added or removed with no comment."  
(or similar, I think doing this when no comment is added might be more 
interesting than being specific to a nosy list modification)

This request again comes from a much applauded feature in our bug 
tracker at work: the ability to tell it when to skip sending email while 
modifying a bug so you avoid spamming people for user-determined 
inconsequential updates.

All actions are still tracked and recorded and visible in issues so 
nobody abuses it for any socially unacceptable purposes (they'd be 
hounded by their peers if they did).