Some people set themselves to "nomail" for an indefinite amount of time (e.g. they no longer want to want to leave, but find it too drastic to unsubscribe, or they want to read via the web but don't want to receive a digest), whereas others do so only for short, set periods of time (e.g. going away on holidays).
Some group owners do not want "sleeping" members in their groups (i.e. members who are on nomail or special notice indefinitely).
So, a suggestion:
Make it possible for members to go nomail for a set period of time only. In other words, when they change their subscription to "nomail", they [can] also set for how many days they want to be nomail, and once that time is over, their subscription automatically reverts to the previous subscription setting (or: to digest).
Also, group owners should be able to *require* that members who want to go nomail, should set a period.
Also (instead of or independent of the previous point), group owners should be able to set a maximum nomail time (if the member wants to be nomail for longer, he has to communicate with the group owner directly to ask for a longer extension).
Also, if a member goes nomail via e-mail and does not specify the length of time that he will be nomail, he should be nomail for only as long as a default amount of nomail duration set by the group owner (which can be the same as the maximum nomail time, or another duration set by the moderator).