ef 70-200 2.8 IS II and 50mm 1.8?

I recently purchased the EF 70-200 2.8 IS II, does it make sense to add the 50mm 1.8 to this (cheap, light weighted)?

Sure why not?

Only if you have a use for it.
What camera do you use? What other lenses do you have? What do you photograph?

50mm lenses are very useful in my opinion. I'd go for it.

No, if you have to ask the question you will be agonising over whether to swap lenses when you're actually out and about photographing.

Your zoom can shoot in lower light than the 50mm f/1.8, due to its 3-stop IS system. Heavy, though.