I think 'bosses' are important in terms of offering players characters to defeat. Part of the reason I found Mass Effect to be a more rewarding story than Mass Effect 2 was because Saren provided this tangible enemy to focus on, instead of 'oh, this weird race and the Reapers...probably'. I don't even remember the boss fight with Saren to be honest, but I remember wanting to beat him.
For me, the most satisfying boss fights I can remember were in the original Soul Reaver. Each of those bosses were Raziel's former friends, who stood idly by while Kain maimed him. I hated all of them and wanted to make them pay. In the time Raziel had been gone, they had all evolved into monstrosities, far too big and powerful for Raziel to simply fight.
So they were basically designed as puzzles, where players had to use the environment against them. It's been done since, but never as well IMO. It didn't feel hackneyed or repeditive or boring, each one was it's one unique, satisfying challenge.