When a solution has been created in a collaborative, multi-iterative group process over a period time, the group's intimacy with the solution, in part, is what makes for success in a session. They understand how the parts make the whole. Basically, they have spent time with the frog. The result is a solution set that is often more innovative and unrepeatable than a solution crafted under more conventional means.
When people ask to be brought into the session late, because of schedule conflicts or other barriers, it is a disincentive to the process - lacking hands on experience and context they need to be "brought up-to-speed". The “bringing them up-to-speed” means taking valuable time and energy going back and explaining and recreating the work up to the moment. It diminishes the energy of the group, as it often takes many people to explain to one. It channels the energy away from the true goal of creating the solution.