This is true and that's why Blizzard moved towards personal legendaries because the amount of people who got their shadowmourne or tarecgosa in a server #10 guild for being the "top performer" and immediately after grabbing it jumped to let's say server #2 guild was creating a lot of drama and tears.
Investing solely in your top performers even when you're 100% certain of their loyalty is still a risk - what if they get a job that suddenly collides with raid times? Get a child born? Get a scholarship far away? Go to a hospital? Now a big chunk of your loot progress is gone with that guy.