Opt-out also gives you more fine-grain access.
You want to opt-out of an entire category, set the category to ignore.
You want to opt-out of specific threads, then you just ignore those threads the same way.
If you want to opt-out of everything in a category except for specific threads, you can go in and set the category to ignore, and then set the specific threads to 'Watching' which will override the category behavior.
It really does give people a hell of a lot more control over their ultimate experience.