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.