Why does Weyland have to hide that he was on the ship?

I got the impression Weyland did not want the scientists (especially the two head scientists) to realize his selfish reason for wanting to contact the engineers. He kept them under the impression that he was funding the mission so that mankind could have the knowledge, even faking that he was already dead, the ultimate way to get credibility.

If he had been honest and said "I want the engineers to help me live forever" the scientists might not have gone along as he wanted them to. (For example, David suggests Weyland wanted Elizabeth Shaw there as a good-luck charm).

Remember the scene where Vickers tells the two scientists that they are not to make contact with the engineers? This is the first hint that Weyland (in the hologram) was lying. Vickers knew Weyland was alive, and knew his true goal, and did not want the scientists to interfere with that goal.