Because he has had so many references to him stuffed into the add-ons that the fanbase will probably explode with rage when they play Lonesome Road and find that both Ulysses and Lonesome Road do not live up to all the hype and references.
Because he is referenced in all(Dead Money not sure) of the dlcs and in the main game, as he saw your name on the list below him and let you deliver the package., possibly knowing that you would get killed. It can be inferred through dialogue with other characters that he is well known and possibly violent/cruel/holds-a grudge-type-character, and that you will have a battle with him.
I guess thats a good enough reason as to why he is important.
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