Roundup should not send email by default when simply adding or removing 
oneself from the Nosy list on an issue.  This mail is generally 
considered spam.  It is *rarely* read and discourages people from adding 
themselves or removing themselves as they know they are spamming the rest 
of the people or mailing lists already listed as 
Nosy/Assigned/ReportedBy.

There's an entry about this in the "Desired tracker features" wiki page
at http://wiki.python.org/moin/DesiredTrackerFeatures (ctrl+f for
"unimportant"), but we haven't reach a consensus yet.
A better approach would be to make notifications configurable, and allow
the user to decide what they want to receive by mails.

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).

A problem with configurable option is that sometimes you want to get
notified by changes in the status (e.g. the issue is closed) or priority
(e.g. becomes high), but you might not care if the nosy changes or a
component/keyword is added/removed.

I initially misread the title of the issue and didn't notice that you
were talking about adding/removing *oneself* only, but the same issue
also applies to mails sent for any minor change.
About your original suggestion, while it could be special-cased, I often
use the email I get while adding myself to the nosy as a remainder that
I should look at the issue.

> the ability to tell it when to skip sending email while 
> modifying a bug so you avoid spamming people for user-determined 
> inconsequential updates.

I like this.  I saw a similar feature in our wiki, where you can mark a
change as "trivial" (e.g. typo fix) and it will be reported only to
persons that have chosen to get notified about trivial changes too.
Having a more explicit "Don't send mail" checkbox would be even better.
 This is somewhat different from configurable options and less flexible,
but it's probably easier to implement and might avoid most of the
"useless" mails.  (This assuming that the person doing the change knows
whether the change is important or not and that all the recipients agree
on that.)

It should be mentioned that this feature should probably be hidden to
regular users tough, since they could remove messages, files, and user
from the nosy list without anyone noticing.  Similarly they could add
spam messages/files and people will not notice them until they go
looking at the issue again.