Heh. I remember when one forum I'm on upgraded from php/bb2 to 3 because they found a security loophole in 2 that you could drive a truck through. (Don't remember what it is, but definitely something that could occur in real life.) The other advantage of 3 is that huge numbers of popular third-party mods from 2 are now built in so you don't have to deal with all the compatibility issues and each module's own potential security flaws.
Problems also arise when you don't or can't keep up to date, because sooner or later the people up top will stop supporting the older version. So don't get something that your present hardware and OS can only barely handle.