I've never seen a carb that was so bad it couldn't be cleaned. And I've torn apart hundreds. Regarding wear on the needle and/or seat - I haven't seen that either. But it's not wear that needs to be worried about, it's corrosion/oxidation.
If even a little of the tapered part of the needle corrodes away in the right spot it will not seal fluid-tight. The other issue I commonly see is a bit of oxidation sticking the needle open - usually this happens where the body of the needle is supposed to slide freely inside the bore in the carb and they get sort of "glued" together. The problem with just cleaning in that situation is that once the corrosion has happened once, it comes back easier and faster the next time. Best to just get the rebuild kit. If you can find a carb that you're SURE is new for a good price that will work too. If you're buying a rebuilt carb you might just as well rebuild your own.
Edit: I should have more properly said I haven't seen needle/seat wear that actually caused leakage. I've definitely seen needles with pretty big grooves worn in them. The thing is, they seem to wear with the seat into a perfect match, so they rarely leak much even when they're pretty worn.
Last edited by eKretz; 04-08-2019 at 12:02 PM.