The current "anon number" system isn't anonymous at all, it's really your ID NUMBER and it's affixed to your username, so if you stay anon all the time, that number really becomes your "identity" as far as other posters are concerned, or if you happen to switch back and forth, other observant members can figure out what number is associated with your name, then it's not really anonymous or even "partially anonymous" at all.
The option to post anonymously should be exactly that - ANONYMOUS. Instead of displaying your user ID in place of your name, it should simply say "Anonymous".
The problem with this is that in a thread with multiple anonymous people, it can be hard to distinguish between the different anonymous posters.
One way to remedy this, although it might be tricky to code, could be to affix an incremental number after "Anonymous" for each new anon person that posts in that thread, then that person retains that appended number for the SCOPE OF THAT THREAD ONLY.
I start a new thread, anonymously. It shows up as being posted by "Anonymous". Then Lapis shows up and starts flaming me (... the nerve!), he will show up as "Anonymous_2", and all of his anonymous posts within THAT THREAD will also show as coming from "Anonymous_2". If another person joins in (you know how hard it is to resist a good flame war!), then their anonymous posts will be as "Anonymous_3".
This would keep people completely anonymous (their ID and username should still be visible to mods though!) while still retaining the ability to distinguish between different anon posters in a thread.
If, in the above example, "Anonymous_3" then created a NEW THREAD, THAT would show up as being created by "Anonymous", and then if I came in and posted under it, I would then be "Anonymous_2" within that thread.