dyscode: As with mostly all DLC it just adds flavour and is totally optional, I'll give you that.
But North Point, Zodiac and Martial Arts are really worthwhile if you like beating up thugs and Ghosts.
And yes, the base game can hold up by itself and no DLC is needed to enjoy the game. And any other DLC than the above is really only worthwhile buying for $1 or so.
But "truly worthless", def. not.
StingingVelvet: The outfits and cars are not worth $1, they are superfluous bullshit. The two "real" mission DLCs are both terrible, from what I have read and watched, and don't mix into the main game at all.
I would call that the definition of worthless, but as always opinions differ.
The Mission are not supposed to mix with the main game, they are each a 'small' homage to common tropes of Eastern Movies.
The Outfits give you certain bonus feats that fall not under cheating, rather support of your favourite playstyle of full contact or guns play.
The cars, yes, they are candy.
Admittedly, you get quickly overpowered with all these DLC.
The DLC in e.g. Space Marine and Dawn Of War: R are much more useless, since these are just Skins without any feats.
Anyway, the choice is yours and your "Your mileage may vary" as usual.