Voight-Kampff: That sound terrible!
A good comparison would be Durlag's Tower in Baldur's Gate. It's an optional area with decent loot, though if your party isn't experienced enough, you will get slaughtered. You can safely skip the area without missing anything from the story, and the game is a complete experience without ever visiting the area. It may add a bit more to the lore and/or setting of the game, but it's not essential.
Omerta's DLCs are that way. They add a bit more lore, they add a bit more loot, but if your party isn't up to the task, you will get slaughtered. You can safely ignore them though without any consequences to the story.
As for the "Incomplete" part, it depends on what you consider complete. How many Star Wars movies must one see before he has seen the complete Star Wars? 1? 3? 6? 9? 250? Must he also watch the series? Read the books? These DLCs are the same way, they expand on the established lore, but are not required. And this does come from a collector/hoarder with an OCPD that makes him want to collect everything.