UPDATE:
Sad to say I've witnessed someone wanting os_mark in #Anope, but when asked what it does the user said "i dunno". They then said they wanted it so they can watch the channel for illegal activities .. etc.
Marking a channel or does *NOT* spy on it. It simply stops other admin other than the admin that marked the channel from using saset, getpass or sendpass on the nick/chan/ It would also stop the nick/channel owner from changing the password as well.
The best example I can think of for usefulness of os mark is in the handling of nick/channel ownership. If you get 2 users that both know the founders password and both lay claim to the channel, a Services admin withou access to Services logs will not be able to tell who the rightful owner is considering how easy it is to use /cs set founder. Granted, Channels are rather protected since CSSecureFounder is enabled by default. OS MARK has other uses as well and nothing is in place for nicks.
OS MARK isn't as usefull on a smaller network as a opposed to a larger one when effective oper communication is harder.