Cookies: some users complained about going from work to home, etc. Some also said that they use multiple computers where they are, and that eliminates cookies as a solution.
You can always use 'show posts since last visit' which works correctly and will show you everything from the real last visit.
Basides the occasional absence, like you indicated which has a good solution, I would think most everyone would frequently visit the forums (at least once every 3 days, or once every 7 days).
For performance reasons, I cannot extend the search to infinite past. It is not going to work for us.
Constructive discussions are good, no reason to hang yourself over them, russel.