Working on the same project for a long time makes you too focused on the details and disconnected from the big picture.
That's why it's so refreshing to have new people join the team. We have someone new at customer service, Michel. He's Micheld in the forums.
When Michel started using AceProject, he got excited at things that we took for granted. It made us realize that we had been focusing on what needed improvement in AceProject, and we didn't notice what worked really well. For example, Michel was impressed with the access rights flexibility when assigning a user to a project.
New people on a project will give the team a new perspective, that's not tainted by time. If only for the inspiring effect, it's worth exchanging project team members every once in a while – think of it as a cultural exchange. The developer from product A goes to work on the product B project, and vice versa. The new team member may be able to solve a problem that had the team stumped, not because she's better, but because she is approaching the problem from a different angle, from outside the project.
Injecting new people into a project team will give it its second wind.