If you try to view the userlist on my forum, all you see is this:
If you have guest viewable forums (ie: they can see posts), your giving away valid usernames. If you (or your users) re-use usernames across different systems, well... same thing.
If you truely want to stop "crackers" (ie: viruses/scrikidios) just implement something with "X fails in Y minute == block for Z minutes"
Normally preventing 5+ log-in with the same username, or from the same IP for 15 minutes at a time will slow down brute force cracking enough to fill your logs with enough info to nail the bugger using up your bandwidth to try thier latest version of 1337autocrak. That's about 480 attempts per day, 3360 per week, if you don't notice this in your logs, re-write your log parsers! If your users have passwords that can be cracked in 3360 attempts, then write a script to brute force thier passwords durring off hours, and force a password reset on the found weak passwords. Or better yet do a spell check on passwords and if a match comes up with less then 3 differences don't allow the password. Then again, unless your working for that secret world goverenment organization running the inter-galactic stargate program, I doubt you'll need this much protection. Monthly stored backups, and weekly backups (rotated monthly) and daily backups (rotated weekly) of your database can restore any damage done to a forum in less time then it takes to go for a pee (women excluded, not trying to be sexist, that just the way it is though ).
I'll admit more security is good security, but one has to understand that some people can't tell left mouse click from reset button on a computer. Limiting easy of use and accessability so that the admin doesn't have to write one more script is lazy IMHO. Course if you want your users to have image verification each time they log on, and force them to have a retinal scan to post be my guest.