(cosmetic crap and some scripted missions that are always the same? No thanks)
Eh, just my opinion but it's actually worth it.
The cosmetic stuff is obviously fluff, but I feel it adds a lot. Being able to customise your soldiers more if nice when you have larger squads (6-10 in Long War) and rosters (my latest run has more than 50 promoted and in rotation).
The scripted missions are actually pretty enjoyable (IMO) and since most one-off maps enter standard rotation for other missions in Long War they add gameplay even if you turn the missions off. The Confounding Light map from Slingshot is one of my favourites. Long War has also messed with the scripting and timing of the missions so they aren't always as predictable. If you were going to play vanilla then Slingshot does decrease the difficulty of the early-mid game somewhat by giving you a strong bonus soldier and huge pile of resources for ploughing through some fun but fairly easy missions (with one notable exception, and many people who would call me a filthy liar after squad wipes...), which may be a positive or negative depending on your view of things.
Honestly, this is a game where I think the more content the better in general, even if just to serve as additional modding hooks. I looked at how the Long War team used the bonus material and realised I should have just bought it all in the first place.