Well, here's a thought as to why you should use different passwords: The people who get the usernames/passwords for the links board normally get them through proxies/etc. - and so if you're using any of those, and they happen to scoop up a username/password from someone who uses the same combination for everything, it wouldn't take a lot of work for an unscrupulous person to go and check various banks and webmail services with that username/password and see if any work.
I'll gladly take a bit of inconvenience (remembering all of my different passwords or using the Password Safe program I linked to above) over having someone break into one of my accounts. Sure, "I don't have anything important" in there, but it's a peace of mind issue - and I'm a network security professional.
