Why register on 800notes.com ?

Sure. Just don't log in (or log out if you are already logged in).

} registered user post anonymously
It's not encouraged but the option exists. I don't know of anyone abusing the feature. You never lose the ability to post while logged off anyway. But realistically the neighborhood is usually safe enough to obviate the most cautious anonymity.

} is there a reason NOT to register?
I suppose people who wish to be less visible, accountable, or transparent may not like it. But then they miss out on the branded tote bag and the disappearing TARDIS beverage mug. Wait, sorry, that's from pledge week on the PBS station.

*I* want a disappearing TARDIS beverage mug!

Here ya go: Disappearing Tardis Mug
Can't help ya with the 800notes tote bag.

That actually looks pretty neat.

"Can a registered user post anonymously on occasion?"  I would have to wonder why you would ask that question.

Guess that person didn't notice the [_] Post anonymously next to their screen name or just thought it was funny to ask.

If you are not signed in you do not see that option, just the "Your Name" box.  "Another Anonymous" was not signed in (assuming they are even registered.)

Cool! Thanks!

Nimrod is correct.  I am not registered so I don't see the option.