Things get messy no matter which way you go. People tend to be a little lazy by nature, and if they can't find the right category that their message fits in they'll just post it anywhere.
But just showing the latest posts makes it difficult for people to come back and look things up. I know there's a search function, but again with the natural laziness of the human critter. People will tend to ask a question rather than taking some action themselves to find an answer.
Sorted by categories with the ability to highlight the most recent posts is the best of both worlds. It tends to be the way most fora work.